Catalytic ammonia synthesis by transition metal molecular complexes

ABSTRACT

This invention relates to molecular catalysts and chemical reactions utilizing the same, and particularly to catalysts and catalytic methods for reduction of molecular nitrogen. The molecular catalytic platform provided herein is capable of the facile reduction of molecular nitrogen under useful conditions such as room temperature or less and atmospheric pressure or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/891,550, filed Oct. 16, 2013, which is hereby incorporated byreference in its entirety to the extent not inconsistent herewith.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under GM070757 awardedby the National Institutes of Health. The government has certain rightsin the invention.

BACKGROUND OF INVENTION

Catalytic conversion of molecular nitrogen to ammonia is a criticalcomponent of the global nitrogen cycle. This chemical transformation iscarried out via a combination of natural biological processes andanthropogenic activities. Biological generation of ammonia via nitrogenreduction occurs primarily by natural processes mediated by organismsresulting in nitrogen fixation necessary for sustaining life. Industrialprocesses for nitrogen reduction, on the other hand, are believed togenerate ammonia on a roughly equivalent scale to the biologicalsources. For example, the annual industrial production of NH₃ vianitrogen reduction is estimated to be approximately 200 million tonnesper year and supports a range of commercial applications includingfertilizers, decontamination and/or sterilization agents, and precursorsfor the production of other nitrogen containing chemicals.

Industrially, the majority of ammonia is produced in commercialquantities via the Haber-Bosch process. This process involves thereaction of nitrogen gas with hydrogen gas in the presence of asolid-state catalyst to produce ammonia. A range of catalysts have beendeveloped for the Haber-Bosh process including iron-based catalysts andruthenium-based catalysts. The Haber-Bosch process is resource intensiveas it typically involves high pressures (˜100 atm), high temperatures(˜450° C.), as well as an industrial supply of precursor hydrogen gas.Given these requirements, ammonia production via industrial processesrepresents a significant amount of total energy consumed throughout theworld each year (estimated to be as much as 1%).

In contrast, the transformation of nitrogen gas to ammonia by biologicalprocesses occurs efficiently under ambient conditions. This process ismediated by organisms, such as diazotrophs, and involves cofactors ofnitrogenase enzymes rich in Fe and S and which may additionally featureMo and V. Substantial research has been directed toward understand thechemical mechanism of the iron-molybdenum cofactor, for example, thatprovides for catalytic conversion of nitrogen into ammonia at lowtemperatures and pressures. Despite extensive research over the lastseveral decades, the exact mechanism for biological reduction ofnitrogen to ammonia remains uncertain.

The development and study of functionally biomimetic catalysts provideuseful tools for understanding the mechanism involved in nitrogenreduction by nitrogenase enzymes. Such functionally biomimetic catalystsalso provide a potential pathway for development of industriallyrelevant molecular catalysts providing for the reduction of N₂ to NH₃.An example of this has been approach is the development andcharacterization of transition metal molecular complexes capable ofbinding and functionalizing N₂ under ambient conditions.

Molecular systems for reduction of N₂ to NH₃ have traditionally focusedon Mo centers given the presence of Mo in the most thoroughly studiediron-molybdenum cofactor. Tri-amido amine Mo and phosphine-pincer Mocomplexes, for example, have been demonstrated to provide moderatecatalytic efficiencies for reduction of N₂ to NH₃ at ambient temperatureand pressures. Attention has also been more recently directed to apotential role of iron as an active N₂ binding site in iron-molybdenumcofactor given the understanding that iron is the only transition metalessential to all nitrogenases. This approach is also supported by recentspectroscopic and biological data implicating iron, as opposed tomolybdenum, as the active site of N₂ binding in the iron-molybdenumcofactor.

It will be appreciated from the foregoing that there is currently a needin the art for improved molecular catalysts and, methods capable of thefacile conversion of nitrogen to ammonia. Specifically, molecularcatalysts are needed for nitrogen reduction providing useful catalyticefficiencies and turnover under conditions less stringent than those inthe conventional Haber-Bosh process.

SUMMARY OF THE INVENTION

This invention relates to molecular catalysts and chemical reactionsutilizing the same, and particularly to catalysts and catalytic methodsfor reduction of molecular nitrogen. The molecular catalytic platformprovided herein is capable of the facile reduction of molecular nitrogenunder useful conditions such as room temperature or less and atmosphericpressure or less.

Provided herein are catalyst compositions, catalytic systems and methodsfor the reduction of molecular nitrogen. Transition metal catalysts,formations and catalytic methods of the invention, for example, exhibituseful catalytic efficiency and with good turnover ratio and reductionproduct yields under conditions supporting a range of industrialapplications, including the production of ammonia. Catalysts of theinvention include molecular complexes comprising iron and cobalt activesites supported by a phosphine ligand, such as a diphosphine ortrisphosphine ligand, for example, a tris(phosphine)borane,tris(phosphinoaryl)borane, tris(phosphinoaryl)alkyl andtris(phosphinoaryl)silyl ligand. Catalytic systems and methods of theinvention are versatile and can be implemented using a range of processconditions and reactants, including a variety of sources of protons andelectrons. Catalytic systems and methods of the invention are compatiblewith approaches and reaction conditions for kinetically driven catalystformation, activation and regeneration steps. In an embodiment, theinvention provides functionally biomimetic Fe or Co metal catalysts andcatalytic methods for efficient conversion of molecular nitrogen toreduction products such as, for example, ammonia (NH₃) and/or hydrazine(H₂H₄).

In an aspect, the invention provides a catalytic process for thereduction of molecular nitrogen (N₂) to generate a reduction product,the process comprising the steps of: contacting a transition metalcatalyst with a source of protons and a source of electrons in thepresence of the molecular nitrogen, thereby generating the reductionproduct; wherein the transition metal catalyst comprises a metal complexcomprising a transition metal atom selected from the group consisting ofFe and Co, and a phosphine ligand (L). In an embodiment, for example,the metal complex comprises a coordination complex. In an embodiment,for example, the phosphine ligand (L) is a diphosphine ligand or atrisphosphine ligand. In an embodiment, multiple atoms or functionalgroups of the ligand independently establish bonds to the metal atom. Inan embodiment, for example, the phosphine ligand is a polydentateligand, such as a bidentate, tridentate or tetradentate ligand. In anembodiment, for example, the metal complex further comprises an N₂group, for example bonded to a central metal atom of the metal complex.

In an embodiment, the transition metal catalyst, a source of protons, asource of electrons and molecular nitrogen are brought in physicalcontact with each other, for example, in a solution phase, a liquidphase, a gas phase or a combination of these. In an embodiment, forexample, the process is carried out in a mixture comprising thetransition metal catalyst, the source of protons, the source ofelectrons and molecular nitrogen, and optionally one or more solventsand/or additives. In an embodiment, for example, the reduction productis NH₃ and/or N₂H₄. In an embodiment the reduction product issubstantially NH₃ (e.g., 90% or greater yield and optionally 99% orgreater yield) or entirely NH₃. In an embodiment, the process occurs ata temperature less than or equal to 25° C. and/or at a pressure lessthan or equal to 1 atmosphere. In an embodiment, the process occurs at atemperature less than 0° C.

In an embodiment, a ligand (L) of the metal complex of the invention hasthe formula (FX1A), (FX1B) or (FX1C):

wherein X¹ is B, C, Si or P; each of L¹, L² and L³ is independently asubstituted or unsubstituted C₁-C₁₀ alkylene, C₃-C₁₀ cycloalkylene,C₅-C₁₀ arylene, or C₅-C₁₀ heteroarylene; each of R¹, R², R³, R⁴, R⁵, R⁶and R⁷ is independently hydrogen or a substituted or unsubstituted C₁-C₈alkyl, C₃-C₈ cycloalkyl, C₅-C₈ aryl, C₅-C₈ heteroaryl, C₁-C₁₈ acyl,C₂-C₈ alkenyl, C₂-C₈ alkynyl, or —P(OR⁸)₂, wherein each R⁸ isindependently H, C₁-C₈ alkyl, C₃-C₈ cycloalkyl, C₅-C₈ aryl or C₅-C₈heteroaryl. In an embodiment, the ligand of the metal complex is neutralor is charged, such as having a charge of −1 or −2.

As will be generally understood by one having skill in the art, theligands described herein participate in bonding with the metal atom ofthe metal complex, for example via electron donating interactions.Therefore, the valencies of atoms of the ligand (L) structures disclosedherein, including in formulas (FX1)-(FX10), are in some cases intendedto be supplemented by the bonding of the ligand to the metal atom of themetal complexes. Bonding to the metal atom can include various atoms andfunctional groups of the ligand including one or more of the phosphorousatoms, central atoms (X¹), linking groups (L¹, L² and L³) and/orterminating groups (R¹, R², R³, R⁴, R⁵, R⁶, R⁷ and R⁸) as shown informulas (FX1)-(FX10). Accordingly, in some embodiments, the ligandstructures and/or metal complex structures as shown herein may beconsidered ionic and/or multivalent structures (e.g., divalent,trivalent, tetravalent, etc.). In some embodiments, for example, aligand of the metal complexes of the invention may be viewed as ananion, for example, an anion having a −1 or −2 charge state.

In an embodiment, for example, a metal complex of the invention furthercomprises N₂ and has the formula (FX2A), (FX2B) or (FX2C):

wherein Z is the transition metal atom.

An important property of the invention is the catalytic nature of thereduction process, for example, wherein the transition metal catalyst isefficiently regenerated in the process and, thus, available tosubsequently react with molecular nitrogen and further generate thereduction product. In an embodiment, for example, regenerated transitionmetal catalyst subsequently participates in further reaction with thesource of protons, source of electrons and molecular nitrogen to achievecatalytic reduction. In an embodiment, the present process ischaracterized by a turnover frequency greater than or equal to 1×10⁻²sec⁻¹. In an embodiment, the present process generates at least 1reduction product equivalents per transition metal catalyst equivalentand for some applications generates at least 2 reduction productequivalents per transition metal catalyst equivalent.

In an embodiment, for example, the process is characterized by catalystactivation and regeneration steps that are capable of kinetic control,for example, via providing acids, reductants and/or additives tokinetically drive catalyst formation, activation, hydrogenation and/orregeneration steps. The present processes are compatible with a range ofdifferent sources of electrons and sources of protons such as acidsincluding HBAr^(F) ₄ (hydro tetrakis[(3,5-trifluoromethyl)phenyl]borate)and reductants including Na and KC₈. In an embodiment, the inventioncomprises the step of providing the source of protons, such as an acid,at a concentration and/or amount capable of kinetically drivingcatalysis formation, activation, hydrogenation and/or regenerationsteps. In an embodiment, the invention comprises the step of providingthe source of electrons at a concentration and/or amount capable ofkinetically driving catalysis formation and/or regeneration. In anembodiment, the invention comprises the step of replenishing theconcentrations and/or amounts of the transition metal catalyst, sourceof protons, source of electrons, molecular nitrogen or combinationsthereof.

In an embodiment, the transition metal catalyst is a metal complexfurther comprises a N₂ group, for example, wherein the transition metalcatalyst is characterized by a metal atom providing for binding andsubsequent functionalization of a N₂ substrate. In an embodiment, forexample, the transition metal catalyst has the formula (FX3): (L)Z(N₂)⁻(FX3), wherein L is the phosphine ligand, and Z is the transition metalatom, and optionally wherein the N₂ is bound to the transition metalatom. In embodiments, the process further comprises the step ofprotonating the transition metal catalyst so as to generate ahydrogenated metal-N₂ complex, such as a hydrogenated metal-N₂ complexcharacterized by one or more N—H bonds. In an embodiment, for example,protonation occurs via contacting the transition metal catalyst with anacid, such as HBAr^(F) ₄ (hydrotetrakis[(3,5-trifluoromethyl)phenyl]borate), HOTf (triflic acid), HX,HBF₄, H(Al(OR)₄) where R can be fluorinated, ArNH₃+X or a combination ofthese, wherein X is a halogen (F, Cl, Br, I). In an embodiment, forexample, the hydrogenated metal-N₂ complex has the formula (FX4):(L)Z(N_(x)H_(y)) (FX4); wherein x is 1 or 2; y is 1, 2, 3, 4 or 5; L isthe phosphine ligand and Z is the transition metal atom. In someembodiments, the hydrogenated metal-N₂ complex, for example havingformula (FX4), is an ion such as an ion having a charge equal to +2, +1,−1 or −2. In an embodiment, for example, the hydrogenated metal-N₂complex has the formula (FX5A) or (FX5B): (L)Z(NH₂) (FX5A) or (L)Z(NH₃)⁺(FX5B); wherein the phosphine ligand and Z is the transition metal atom.In an embodiment, the catalytic process further comprises reductiveprotonation of the hydrogenated metal-N₂ complex, for example using Na,KC₈, Na/Hg, NaBH₄ ⁻, Mg, Zn or any combination of these, therebygenerating the reduction product and regenerating the transition metalcatalyst.

The invention is further inclusive of transition metal catalystprecursors for generating the present transition metal catalyst. In anembodiment, for example, the invention provides methods wherein atransition metal catalyst precursors is contacted with molecularnitrogen to generate the transition metal catalyst. In an embodiment,for example, the catalytic process further comprises: (i) providing atransition metal precursor comprising a precursor transition metalcomplex comprising the transition metal atom and the phosphine ligand(L); and (ii) contacting the transition metal catalyst precursor withmolecular nitrogen in the presence of an acid and reductant, therebygenerating the transition metal catalyst comprising a N₂ adduct of thetransition metal catalyst precursor. In an embodiment, the transitionmetal catalyst precursor has the formula (L)Z⁺ (FX6), wherein L is thephosphine ligand and Z is the transition metal atom. In an embodiment,the transition metal catalyst is generated via reduction of thetransition metal catalyst precursor, wherein the reducing agent is Na,Na/Hg or KC₈.

The transition metal atom, as well as the axial donor atom within theligand to which it is bonded, may play an important role in certaincatalysts and catalytic methods of the invention. In an embodiment, forexample, an N₂ is directly bonded to the transition metal atom, therebyforming an N₂ adduct. In an embodiment, the transition metal catalystfurther comprises one or more counter ions. A wide range of counter ionsare useful in the present compositions and methods, for example, Na⁺ orK⁺, optionally solvated or coordinated by a crown, cryptand, or anydonor group in principle.

In an embodiment, for example, the transition metal catalyst is amononuclear metal complex. In an embodiment, for example, the transitionmetal catalyst is a metal coordination complex characterized by acoordination number of 3, 4, 5 or 6, and in some embodiments acoordination number of 4 or 5. In an embodiment, the transition metalatom of the transition metal catalyst is Fe. In an embodiment, thetransition metal atom of the transition metal catalyst is Fe andcharacterized by an oxidation state of Fe(−1), Fe(0), Fe(I), Fe(II),Fe(III), or Fe(IV). For example, an LFe(N) precatalyst can have anoxidation state of Fe(IV), and can be an intermediate of catalysis. Inan embodiment, the transition metal atom of the transition metalcatalyst is Co.

In an embodiment, the phosphine ligand is a polydentate ligand, such asa tridentate or tetradentate ligand. In an embodiment, for example, thephosphine ligand is a tripodal trisphospine ligand having a boron,carbon, silicon and/or or phosphorous axial donor atom(s). In anembodiment, the phosphine ligand has an aryl backbone comprising atleast one of L¹, L² and L³ independently comprising C₅-C₁₀ arylene orC₅-C₁₀ heteroarylene. In an embodiment, for example, the phosphineligand comprises one or more cyclohexylamine ring systems.

The invention includes transition metal catalysts comprising a metalcomplex, wherein at least one of, and optionally all of, L¹, L², L³, R¹,R², R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are substituted with one or moreheteroatoms or heavy atom analogues. In an embodiment, for example, atransition metal catalysts comprises a metal complex, wherein at leastone of, and optionally all of, L¹, L² and L³ is independentlysubstituted with one or more heteroatoms, such as one or moreheteroatoms selected from the group consisting of O, N, or Si. In anembodiment, for example, a transition metal catalyst comprises a metalcomplex, wherein at least one of, and optionally all of, R¹, R², R³, R⁴,R⁵, R⁶, R⁷ and R⁸ is independently substituted with one or moreheteroatoms, such as one or more heteroatoms selected from the groupconsisting of O, N, or Si, or heavy atom analogues, such as As.

In an embodiment, for example, the ligand of the transition metalcatalyst has the formula (FX7A), (FX7B) or (FX7C):

wherein X¹, R¹, R², R³, R⁴, R⁵, R⁶ and R⁷, are as set forth in formulas(FX1A), (FX1B) and (FX1C). In an embodiment, the ligand of the metalcomplex is neutral or is charged, such as having a charge of −1 or −2.In an embodiment, for example, the metal complex further comprises N₂and has the formula (FX8A), (FX8B) or (FX8C):

wherein Z is the transition metal atom and wherein X¹, R¹, R², R³, R⁴,R⁵, R⁶ and R⁷, are as set forth in formulas (FX1A), (FX1B) and (FX1C).

In an embodiment, for example, the ligand of the transition metalcatalyst has the formula (FX9A), (FX9B) or (FX9C):

wherein R¹, R², R³, R⁴, R⁵, and R⁶ are as set forth in formulas (FX1A),(FX1B) and (FX1C). In an embodiment, for example, each of R¹, R², R³,R⁴, R⁵, R⁶ and R⁷ is independently isopropyl, phenyl or cyclohexyl. Inan embodiment, for example, one of R¹ and R², R³ and R⁴, and R⁵ and R⁶is different from the others. In an embodiment, the ligand of the metalcomplex is neutral or is charged, such as having a charge of −1 or −2.

In an embodiment, for example, the ligand of the transition metalcatalyst has formula (FX10A), (FX10B) or (FX10C):

wherein iPr is isopropyl, Ph is phenyl, and Cy is cyclohexyl. In anembodiment, for example, the ligand of the transition metal catalyst hasthe formula (FX10A), (FX10B), (FX10C) or (FX10D):

wherein iPr is isopropyl, Ph is phenyl and Cy is cyclohexyl. In anembodiment, the ligand of the metal complex is neutral or is charged,such as having a charge of −1 or −2.

In an embodiment, the transition metal catalyst has the formula[(TP^(R)B)Fe(N₂)]⁻, [(CP^(R) ₃)Fe(N₂)]⁻, [(SiP^(R) ₃)Fe(N₂)]⁻,[(TP^(R)B)Co(N₂)]⁻, [(CP^(R) ₃)Co(N₂)]⁻, or [(SiP^(R) ₃)Co(N₂)]⁻,wherein TP^(R)B is a tris(phosphinoaryl)borane ligand, CP^(R) ₃ is atris(phosphinoaryl)alkyl ligand and SiP^(R) ₃ istris(phosphinoaryl)silyl ligand.

An important feature of the invention is the capability of the catalystsand catalytic systems to reduce molecular nitrogen at ambienttemperatures or lower and atmospheric pressures or lower using aversatile range of reaction conditions. In an embodiment, for example,the transition metal catalyst and the molecular nitrogen are eachdissolved in a solvent; wherein the step of contacting the transitionmetal catalyst with the source of protons and the source of electronsoccurs within a solution. Criteria for selection of an appropriatesolvent include stability to the reductant and the acid used. In anembodiment, for example, the solution is a nonaqueous solution. In anembodiment, the solvent is selected from the group consisting of:diethyl ether, related ethereal solvent (such as tetrahydrofuran ordimethoxyethane or diglym), or an aromatic or alkane solvent such asmethylcyclohexane, or mixtures thereof. In some embodiments, forexample, the solvent has heteroatoms other than oxygen (including thosewith N or S).

In an embodiment, for example, at least one of the molecular nitrogenthe transition metal catalyst, the source of protons and the source ofelectrons, and optionally all of, are provided in a solution comprisingone or more solvents. In an embodiment, the one or more solvents is oneor more nonaqueous solvents. In an embodiment, for example, thetransition metal catalyst is a homogeneous catalyst, wherein thetransition metal catalyst, the source of protons, the source ofelectrons and the molecular nitrogen are provided in contact in thesolution. In an embodiment, for example, the transition metal catalystis a heterogeneous catalyst, where the transition metal, the source ofprotons and the source of electrons and the molecular nitrogen areprovided in the solution and provided in contact with the transitionmetal catalyst provided in the solid phase. In an embodiment, forexample, the transition metal catalyst is a heterogeneous catalyst,where the source of protons and the source of electrons and themolecular nitrogen are provided in the solution, the molecular nitrogenis provided in the gas phase, which are provided in contact with thetransition metal catalyst provided in the solid phase.

In an embodiment, for example, the concentration of the transition metalcatalyst in the solution is selected from the range of 0.01 mM to 10 mM.In an embodiment, for example, the concentration of the molecularnitrogen in the solution is selected from the range of 1×10⁻⁴ M to 1 M.

A wide variety of proton sources is useful in the present invention,including a range of acids. In an embodiment, the source of protons isone or more acids, for example, one or more acids selected from thegroup consisting of: HBAr^(F) ₄ (hydrotetrakis[(3,5-trifluoromethyl)phenyl]borate), HOTf (triflic acid), HX,HBF₄, H(Al(OR)₄) where R can be fluorinated, ArNH₃+X or a combination ofthese; wherein X is a halogen (e.g., F, Cl, Br, or I). In an embodiment,the concentration of the one or more acids is selected from the range of0.01-5 M.

A wide variety of electron sources is useful in the present invention,including a range of reductants. In embodiments, the source of electronsis one or more reductants, for example, one or more reductants selectedfrom the group consisting of Na, KC₈, Na/Hg, NaBH₄ ⁻, Mg, Zn or anycombination of these. In an embodiment, the concentration of the one ormore reductants is selected from the range of 0.1-100 M.

In an embodiment, the transition metal catalyst is provided as ahomogeneous catalyst, for example, wherein the catalyst and molecularnitrogen are provided, and provided in contact with each other, in thesame phase, such as in solution. Alternative, the present transitionmetal catalysts may be provided as heterogeneous catalyst, for example,wherein the catalyst is provided as a phase different from the molecularnitrogen, such as wherein the transition metal catalyst is provided as asolid and contacted by molecular nitrogen in gas or solution phase.Examples of solvents useful for homogeneous or heterogeneous catalyst ofthe invention include nonaqueous solvents such as diethyl ether, diethylether, tetrahydrofuran, dimethoxyethane, diglym, methylcyclohexane orany combination of these.

The catalysts and catalytic processes disclosed herein are efficient andprovide benefits over conventional functional biomimetic catalystsystems for the reduction of molecular nitrogen. In an embodiment, thecatalytic process generates at least 1, and optionally 2 or 10,reduction product equivalents per transition metal catalyst equivalent.

In an aspect, the invention provides a catalyst formulation forreduction of molecular nitrogen (N₂) to generate a reduction product,the formulation comprising: (i) a transition metal catalyst comprising ametal complex comprising a transition metal atom selected from the groupconsisting of Fe and Co, and a phosphine ligand (L); (ii) a source ofprotons comprising one or more acids; and (iii) a source of electronscomprising one or more reductants. In an embodiment, the metal complexof the transition metal catalyst further comprises a N₂ group, forexample, an N₂ group bound to the metal atom. In an embodiment, forexample, the catalyst formulation is provided in solution, for example,further comprising a nonaqueous solvent. As will be generally understoodby those having skill in the art, any of the transition metal catalysts,metal complexes, transition metal catalyst precursors, sources ofelectrodes, sources of protons and solvents described herein can be usedin the present catalyst formulations. In an embodiment, for example theacid is HBAr^(F) ₄ (hydro tetrakis[(3,5-trifluoromethyl)phenyl]borate).In an embodiment, for example, the one or more reductants are selectedfrom the group consisting of Na, KC₈, Na/Hg, NaBH₄ ⁻, Mg, Zn or anycombination of these. In an embodiment, the catalyst formulation of theinvention is in a substantially purified state.

In another aspect, the present invention provides metal complexcompositions, for example, for applications in chemical synthesis. In anembodiment, the invention provides a metal complex comprising atransition metal atom selected from the group consisting of Fe and Co,and a phosphine ligand (L); wherein the ligand (L) has the formula(FX1A), (FX1B) or (FX1C):

wherein X¹ is B, C, Si or P; each of L¹, L² and L³ is independently asubstituted or unsubstituted C₁-C₁₀ alkylene, C₃-C₁₀ cycloalkylene,C₅-C₁₀ arylene, or C₅-C₁₀ heteroarylene; each of R¹, R², R³, R⁴, R⁵, R⁶and R⁷ is independently hydrogen or a substituted or unsubstituted C₁-C₈alkyl, C₃-C₈ cycloalkyl, C₅-C₈ aryl, C₅-C₈ heteroaryl, C₁-C₁₈ acyl,C₂-C₈ alkenyl, C₂-C₈ alkynyl, or —P(OR⁸)₂, wherein each R⁸ isindependently H, C₁-C₈ alkyl, C₃-C₈ cycloalkyl, C₅-C₈ aryl or C₅-C₈heteroaryl with the proviso that when the ligand (L) istris(o-phosphinoaryl)boran, then the metal atom is Co. In an embodiment,the metal complex further comprises a N₂ group, for example, an N₂ groupbound to the metal atom. In an embodiment, for example, the metalcomplex has the formula: (L)Z(N₂)⁻ (FX3). In an embodiment, for example,the invention provides a metal complex having formula (FX1A), (FX1B) or(FX1C), wherein the metal atom is Fe. In an embodiment, for example, theinvention provides a metal complex having formula (FX1A), (FX1B) or(FX1C), wherein the metal atom is Co. In an embodiment, for example, theinvention provides a metal complex having formula (FX1A), (FX1B) or(FX1C), wherein X¹ is C or Si. In an embodiment, for example, theinvention provides a metal complex having formula (FX1A), (FX1B) or(FX1C), wherein at least one of, and optionally all of, R¹, R², R³, R⁴,R⁵, R⁶ and R⁷ is not isopropyl. In an embodiment, for example, theinvention provides a metal complex having formula (FX1A), (FX1B) or(FX1C), wherein at least one of, and optionally all of, R¹, R², R³, R⁴,R⁵, R⁶ and R⁷ is phenyl and cyclohexyl. In an embodiment, the metalcomplex of the invention is in a substantially purified state.

Without wishing to be bound by any particular theory, there may bediscussion herein of beliefs or understandings of underlying principlesrelating to the devices and methods disclosed herein. It is recognizedthat regardless of the ultimate correctness of any mechanisticexplanation or hypothesis, an embodiment of the invention cannonetheless be operative and useful.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B. Schematics of catalytic cycle. Schematic depictions ofcatalytic cycles involving transition metal catalysts of the invention.

FIG. 2. Chemical line representations of the FeMo-cofactor ofnitrogenase. A schematic depiction of N₂ binding and reduction at an Fesite via limiting alternating (top) and distal (bottom) mechanisms. Thedrawing emphasizes a possible hemi-labile role for the interstitialC-atom with respect to an Fe—N₂ binding site.

FIG. 3. Stoichiometric (TPB)Fe—N₂ model reactions. (a) N₂ binding to[(TPB)Fe][BAr^(F) ₄] under electron-loading to generate[(TPB)Fe(N₂)][Na(12-crown-4)₂]. (b) Reductive protonolysis of(TPB)Fe(NH₂) to release NH₃ with concomitant N₂ uptake. (c) Generationof [(TPB)Fe(NH₃)][BAr^(F) ₄] and other (TPB)Fe-species upon addition ofacid to [(TPB)Fe(N₂)][Na(12-crown-4)₂] at low temperature, followed bywarming and then addition of base. 12-C-4 is an abbreviation for12-crown-4. Note: TPB=tris(phosphine)borane.

FIG. 4. Spectral data for ammonia analysis, and catalyst poisoning. (a)¹H NMR spectrum (300 MHz) of [¹⁴NH₄][Cl] in DMSO-d₆ produced from acatalytic run under ¹⁴N₂ (top) and of [¹⁵NH₄][Cl] in DMSO-d₆ producedfrom an independent catalytic run under 1 atm ¹⁵N₂. (b) Representativeoptical data for NH₃ product analysis using the indophenol method fromcatalytic runs using the standard conditions with the precursorsindicated. (c) H₂ binds to (TPB)Fe(N₂) to generate (TPB)(μ-H)Fe(N₂)(H),which itself is ineffective for the catalytic generation of NH₃ from N₂under the standard conditions. Note: TPB=tris(phosphine)borane;DMSO=dimethylsulfoxide.

FIG. 5. ¹H NMR spectrum of the reaction mixture following protonation of[(TPB)Fe(N₂)][Na(12-crown-4)₂] compared with an authentic sample of[(TPB)Fe(NH₃)][BAr^(F) ₄].

FIG. 6. Stacked EPR spectra at 77 K of [(TPB)Fe(N₂)][Na(12-crown-4)₂],the yellow species generated upon addition of acid to[(TPB)Fe(N₂)][Na(12-crown-4)₂], and the green-yellow species generatedfrom (TPB)Fe≡N(p-C₆H₄OMe) and [Fc][BAr^(F) ₄] at 77 K.

FIG. 7. ¹H NMR spectrum of [¹⁴NH₄][Cl] produced from[(TPB)Fe(N₂)][Na(12-crown-4)₂], HBAr^(F) ₄. 2 Et₂O, and KC₈ under ¹⁴N₂.

FIG. 8. ¹H NMR spectrum of [¹⁵NH₄][Cl] produced from[(TPB)Fe(N₂)][Na(12-crown-4)₂], HBAr^(F) ₄.2 Et₂O, and KC₈ under ¹⁵N₂.

FIG. 9. Calibration curves for NH₃ and N₂H₄ UV-Vis quantification.

FIG. 10. IR spectra of addition of 10 equiv HBAr^(F) ₄.2 Et₂O to[(TPB)Fe(N₂)][Na(12-crown-4)₂], followed by 12 equiv KC₈.

FIG. 11. ³¹P{¹H} NMR spectra of addition of 10 equiv HBAr^(F) ₄.2 Et₂Oto [(TPB)Fe(N₂)][Na(12-crown-4)₂], followed by 12 equiv KC₈.

FIG. 12. (Top) Structure of the FeMo cofactor of nitrogenase, showing aputative site for dinitrogen binding and highlighting the trigonalbipyramidal coordination environment at Fe. Possible sites of H-atoms oncofactor prior to N₂ binding not shown. (Bottom) Possible role of Lewisacidic (LA) or aryl substituents in stabilizing ionic character in theN₂—Fe—C_(alkyl) interaction.

FIG. 13. Select trigonal bipyramidal scaffolds and the (CP^(iPr) ₃)FeN₂⁻ system.

FIG. 14. Crystal structures of {(CP^(iPr) ₃)H}FeI₂ (6, top left),{(CP^(iPr) ₃)H}FeBr (8, top right), and (CP^(iPr) ₃)Fe(H)(N₂) (9,bottom). Ellipsoids shown at 50% probability; hydrogen atoms (except thetriarylmethine C—H and Fe—H hydride) and solvent molecules omitted forclarity.

FIG. 15. Crystal structures of (CP^(iPr) ₃)FeN₂ (11, left), (CP^(iPr)₃)FeN₂ ⁻ (12[K(Et₂O)₃], center, ethyl groups of coordinated Et₂Omolecules omitted), and (CP^(iPr) ₃)FeN₂ ⁺ (13, right). Ellipsoids areshown at 50% probability and hydrogen atoms are omitted for clarity.

FIG. 16. Cyclic voltammogram of 11; scan rate 0.5 V/s.

FIG. 17. (A) Isocontour plot of the Fe—C_(alkyl) σ bond of 12[K(Et₂O)₃]located from NBO analyses. (B) Contour plot of one of the C_(aryl) π*orbitals which accepts delocalized electron density from theFe—C_(alkyl) σ bond.

FIG. 18. Spectroscopic analyses of reaction mixtures following thecatalytic production of NH₃ using 12[K(Et₂O)_(0.5)] as a catalyst.Symbols indicate characteristic resonances attributed to 9, 10, and 11.(A),(C) ¹H NMR and IR spectra of a post-catalytic reaction mixture using10 equiv. of [H(Et₂O)₂][BAr^(F) ₄] and 12 equiv. of KC₈. (B),(D) ¹H NMRand IR spectra of a post-catalytic reaction mixture using 38 equiv. of[H(Et₂O)₂][BAr^(F) ₄] and 40 equiv. of KC₈.

FIG. 19. ¹H NMR of 2 (DMSO-d₆, 300 MHz).

FIG. 20. ¹³C NMR of 2 (DMSO-d₆, 75 MHz).

FIG. 21. ¹H NMR of 3 (CDCl₃, 300 MHz).

FIG. 22. ¹³C NMR of 3 (CDCl₃, 75 MHz).

FIG. 23. ¹H NMR of 4 (aromatic region) in DMSO-d₆ (300 MHz).

FIG. 24. ¹H NMR of 5 (CDCl₃, 300 MHz).

FIG. 25. ¹³C NMR of 5 (CDCl₃, 75 MHz).

FIG. 26. ¹H NMR of 1 (C₆D₆, 300 MHz) with inset showing coupling tocentral methine proton.

FIG. 27. ¹³C NMR of 1 (C₆D₆, 75 MHz).

FIG. 28. ³¹P NMR of 1 (C₆D₆, 300 MHz).

FIG. 29. ¹H NMR of 6 (C₆D₆) with inset showing peak at 180 ppm. No otherpeaks appear outside the range depicted.

FIG. 30. ¹H NMR of 7 (C₆D₆, 300 MHz), unpurified, generated by reductionof 6 with sodium amalgam in benzene.

FIG. 31. ¹H NMR of 9 (C₆D₆, 300 MHz) with inset showing ³¹P NMR.

FIG. 32. IR (thin film deposited from benzene) of 9; v(NN)=2046 cm⁻¹;v(FeH)=1920 cm⁻¹.

FIG. 33. ¹H NMR of 8 (C₆D₆, 300 MHz), unpurified, generated by reductionof CP3HFeBr2 with isopropyl magnesium chloride in toluene. For unknownreasons the reaction appears to yield a mixture of two closely relatedparamagnetic species.

FIG. 34. ¹H NMR of 10 (C₆D₆, 300 MHz).

FIG. 35. ¹H NMR of 11 (C₆D₆, 300 MHz).

FIG. 36. IR (thin film deposited from benzene) of 11. v(NN)=1992 cm⁻¹.

FIG. 37. ¹H NMR of 12 (d⁸-THF, 300 MHz).

FIG. 38. ³¹P NMR of 12.

FIG. 39. IR spectrum of 12 as a thin film deposited fromdimethoxyethane.

FIG. 40. ¹H NMR of 12[K(12-crown-4)₂] in d₈-THF.

FIG. 41. IR spectrum of 12[K(12-crown-4)₂] (thin film from THF).

FIG. 42. ¹H NMR (4:1 C₆D₆/THF-d₈ under N₂, 300 MHz, 298 K) of 13.

FIG. 43. IR of 13 as a thin film deposited from THF.

FIG. 44. UV-Vis spectra of 13 under N₂ and under static vacuum (afterthree freeze-pump-thaw cycles). 13 is in a solution of 3:1 Et₂O:THF at aconcentration of 0.54 mM.

FIG. 45. ¹H NMR of 14, C₆D₆.

FIG. 46. ¹H NMR of 14, showing paramagnetic regions magnified to showminor contamination with CP^(iPr) ₃FeN₂ (11).

FIG. 47. ³¹P NMR of 14 (C₆D₆).

FIG. 48. IR (thin-film from C₆D₆) of 14. Peak at 1992 cm⁻¹ iscontamination by CP^(iPr) ₃FeN₂.

FIG. 49. NMR of reaction mixture after catalysis with internal standard(1,3,5-trimethoxybenzene).

FIG. 50. Calibration curve for NH₃ quantification via indophenol method.

FIG. 51. Calibration curve for UV-Vis quantification of hydrazine.

FIG. 52. X-Ray Diffraction (XRD) structures of complexes 1 (A) and 2 (B)with hydrogen atoms and counterion (for B) omitted for clarity. SeeTable 24 for selected bond lengths and angles.

FIG. 53. XRD structures of the cores of complexes 3 (A), 4 (B), and 5(C). See Table 24 for selected distances and angles.

FIG. 54. X-Band EPR spectra for complexes 1-6. Conditions for 1:Toluene, 8 K; 2: 2:1 Toluene:Et₂O, 10 K 3: 2-MeTHF, 10 K; 4: 2-MeTHF, 10K; 5: 2-MeTHF, 10 K; 6: Toluene, 10 K.

FIG. 55. ¹H NMR Spectrum of (TPB)FeMe (1).

FIG. 56. ¹H NMR Spectrum of [(TPB)Fe][BAr^(F) ₄] (2).

FIG. 57. ¹H NMR Spectrum of [(TPB)Fe(N₂H₄)][BAr^(F) ₄] (3).

FIG. 58. ¹H NMR Spectrum of [(TPB)Fe(NH₃)][BAr^(F) ₄] (4).

FIG. 59. ¹H NMR Spectrum of (TPB)FeNH₂ (5).

FIG. 60. ¹H NMR Spectrum of (TPB)FeOH (6).

FIG. 61. 10 K EPR Spectrum of (TPB)FeMe (1).

FIG. 62. 10 K EPR Spectrum of [(TPB)Fe][BAr^(F) ₄] (2).

FIG. 63. 10 K EPR Spectrum of [(TPB)Fe(N₂H₄)][BAr^(F) ₄] (3).

FIG. 64. 10 K EPR Spectrum of [(TPB)Fe(NH₃)][BAr^(F) ₄] (4).

FIG. 65. 10 K EPR Spectrum of (TPB)FeNH₂ (5).

FIG. 66. 10 K EPR Spectrum of (TPB)FeOH (6).

FIG. 67. UV-Vis Spectrum of (TPB)FeMe (1).

FIG. 68. UV-Vis Spectrum of [(TPB)Fe][BAr^(F) ₄] (2).

FIG. 69. UV-Vis Spectrum of [(TPB)Fe(N₂H₄)][BAr^(F) ₄] (3).

FIG. 70. UV-Vis Spectrum of [(TPB)Fe(NH₃)][BAr^(F) ₄] (4).

FIG. 71. UV-Vis Spectrum of (TPB)FeNH₂ (5).

FIG. 72. UV-Vis Spectrum of (TPB)FeOH (6).

FIG. 73. Titration of THF into an ethereal solution of 2.

FIG. 74. NMR traces of the monitored decomposition of 3 to 4.

FIG. 75. Kinetic plots of the monitored decomposition of 3 to 4.

FIG. 76. Geometries of [(TPB)Fe]⁺ and [(Me₂PhP)₃Fe]⁺ optimized at theB3LYP/6-31 G(d) level.

FIG. 77. MO and spin density diagram of [(TPB)Fe]⁺ optimized at theB3LYP/6-31 G(d) level.

FIG. 78. MO diagram offering a tentative explanation for the T-shapeddistortion of 2.

FIG. 79. Variable Temperature Magnetic data for 2-5.

FIG. 80. Crystal Structure for (TPB)FeOH (6).

FIG. 81. Complexes synthesized and studied towards N₂ reduction in thiswork.

FIG. 82. XRD structures of complexes 1 (E), 3 (D), 5 (A), 8 (B), 9 (C),and 10 (F) with ellipsoids at 50% and hydrogens and counterions omittedfor clarity. Fe atoms are shown in orange, P in purple, B in tan, Cl ingreen, and N in blue. See Table 32 for bonding metrics.

FIG. 83. Catalytic competence and vibrational spectroscopy data forselect (P₃X)M(N₂)⁻ complexes. Data for M=Fe, X=B is from refs. 8 and 11;data for M=Fe, X=C is from ref. 9; data for M=Fe, X=Si is from ref.s 9and 13; and data for M=Co, X=B is from this work. ^(a)IR of a KBr pellet^(b)IR of a thin film from evaporation of solvent.

FIG. 84. Calibration curve for NH₃ quantification by indophenol method.

FIG. 85. Calibration curve for UV-vis quantification of hydrazine.

FIG. 86. Cyclic voltammagram of (TPB)Co(N₂) (1) scanning oxidatively(left) and reductively (right) at 100 mV/sec in THF with 0.1 M TBAPF₆electrolyte.

FIG. 87. Temperature dependence of the magnetic susceptibility of[(TPB)Co][BAr^(F) ₄] (3) as measured by SQUID magnetometry.

STATEMENTS REGARDING CHEMICAL COMPOUNDS AND NOMENCLATURE

In an embodiment, a composition or compound of the invention, such as ametal catalyst composition or formulation, is isolated or substantiallypurified. In an embodiment, an isolated or purified compound is at leastpartially isolated or substantially purified as would be understood inthe art. In an embodiment, a substantially purified composition,compound or formulation of the invention has a chemical purity of 95%,optionally for some applications 99%, optionally for some applications99.9%, optionally for some applications 99.99%, and optionally for someapplications 99.999% pure.

Many of the molecules disclosed herein contain one or more ionizablegroups. Ionizable groups include groups from which a proton can beremoved (e.g., —COOH) or added (e.g., amines) and groups that can bequaternized (e.g., amines). All possible ionic forms of such moleculesand salts thereof are intended to be included individually in thedisclosure herein. With regard to salts of the compounds herein, one ofordinary skill in the art can select from among a wide variety ofavailable counterions that are appropriate for preparation of salts ofthis invention for a given application. In specific applications, theselection of a given anion or cation for preparation of a salt canresult in increased or decreased solubility of that salt.

The compounds of this invention can contain one or more chiral centers.Accordingly, this invention is intended to include racemic mixtures,diasteromers, enantiomers, tautomers and mixtures enriched in one ormore stereoisomer. The scope of the invention as described and claimedencompasses the racemic forms of the compounds as well as the individualenantiomers and non-racemic mixtures thereof.

As used herein, the term “group” may refer to a functional group of achemical compound. Groups of the present compounds refer to an atom or acollection of atoms that are a part of the compound. Groups of thepresent invention may be attached to other atoms of the compound via oneor more covalent bonds. Groups may also be characterized with respect totheir valence state. The present invention includes groups characterizedas monovalent, divalent, trivalent, etc. valence states.

As used herein, the term “substituted” refers to a compound wherein ahydrogen is replaced by another functional group.

As is customary and well known in the art, hydrogen atoms in formulas(FX1)-(FX10) are not always explicitly shown, for example, hydrogenatoms bonded to the carbon atoms of alkylene groups and/or alicyclicrings are not always explicitly shown in formulas (FX1)-(FX10). Thestructures provided herein, for example in the context of thedescription of formulas (FX1)-(FX10), are intended to convey to one ofreasonable skill in the art the chemical composition of compounds of themethods and compositions of the invention, and as will be understood byone of skill in the art, the structures provided do not indicate thespecific positions of atoms and bond angles between atoms of thesecompounds.

Alkyl groups include straight-chain, branched and cyclic alkyl groups.Alkyl groups include those having from 1 to 30 carbon atoms. Alkylgroups include small alkyl groups having 1 to 3 carbon atoms. Alkylgroups include medium length alkyl groups having from 4-10 carbon atoms.Alkyl groups include long alkyl groups having more than 10 carbon atoms,particularly those having 10-30 carbon atoms. The term cycloalkylspecifically refers to an alky group having a ring structure such asring structure comprising 3-30 carbon atoms, optionally 3-20 carbonatoms and optionally 2-10 carbon atoms, including an alkyl group havingone or more rings. Cycloalkyl groups include those having a 3-, 4-, 5-,6-, 7-, 8-, 9- or 10-member carbon ring(s) and particularly those havinga 3-, 4-, 5-, 6-, 7- or 8-member ring(s). The carbon rings in cycloalkylgroups can also carry alkyl groups. Cycloalkyl groups can includebicyclic and tricycloalkyl groups. Alkyl groups are optionallysubstituted. Substituted alkyl groups include among others those whichare substituted with aryl groups, which in turn can be optionallysubstituted. Specific alkyl groups include methyl, ethyl, n-propyl,iso-propyl, cyclopropyl, n-butyl, s-butyl, t-butyl, cyclobutyl,n-pentyl, branched-pentyl, cyclopentyl, n-hexyl, branched hexyl, andcyclohexyl groups, all of which are optionally substituted. Substitutedalkyl groups include fully halogenated or semihalogenated alkyl groups,such as alkyl groups having one or more hydrogens replaced with one ormore fluorine atoms, chlorine atoms, bromine atoms and/or iodine atoms.Substituted alkyl groups include fully fluorinated or semifluorinatedalkyl groups, such as alkyl groups having one or more hydrogens replacedwith one or more fluorine atoms. Substituted alkyl groups may includesubstitution to incorporate one or more silyl groups, for examplewherein one or more carbons are replaced by Si.

An alkoxy group is an alkyl group that has been modified by linkage tooxygen and can be represented by the formula R—O and can also bereferred to as an alkyl ether group. Examples of alkoxy groups include,but are not limited to, methoxy, ethoxy, propoxy, butoxy and heptoxy.Alkoxy groups include substituted alkoxy groups wherein the alky portionof the groups is substituted as provided herein in connection with thedescription of alkyl groups. As used herein MeO— refers to CH₃O—.

Alkenyl groups include straight-chain, branched and cyclic alkenylgroups. Alkenyl groups include those having 1, 2 or more double bondsand those in which two or more of the double bonds are conjugated doublebonds. Alkenyl groups include those having from 2 to 20 carbon atoms.Alkenyl groups include small alkenyl groups having 2 to 3 carbon atoms.Alkenyl groups include medium length alkenyl groups having from 4-10carbon atoms. Alkenyl groups include long alkenyl groups having morethan 10 carbon atoms, particularly those having 10-20 carbon atoms.Cycloalkenyl groups include those in which a double bond is in the ringor in an alkenyl group attached to a ring. The term cycloalkenylspecifically refers to an alkenyl group having a ring structure,including an alkenyl group having a 3-, 4-, 5-, 6-, 7-, 8-, 9- or10-member carbon ring(s) and particularly those having a 3-, 4-, 5-, 6-,7- or 8-member ring(s). The carbon rings in cycloalkenyl groups can alsocarry alkyl groups. Cycloalkenyl groups can include bicyclic andtricyclic alkenyl groups. Alkenyl groups are optionally substituted.Substituted alkenyl groups include among others those that aresubstituted with alkyl or aryl groups, which groups in turn can beoptionally substituted. Specific alkenyl groups include ethenyl,prop-1-enyl, prop-2-enyl, cycloprop-1-enyl, but-1-enyl, but-2-enyl,cyclobut-1-enyl, cyclobut-2-enyl, pent-1-enyl, pent-2-enyl, branchedpentenyl, cyclopent-1-enyl, hex-1-enyl, branched hexenyl, cyclohexenyl,all of which are optionally substituted. Substituted alkenyl groupsinclude fully halogenated or semihalogenated alkenyl groups, such asalkenyl groups having one or more hydrogens replaced with one or morefluorine atoms, chlorine atoms, bromine atoms and/or iodine atoms.Substituted alkenyl groups include fully fluorinated or semifluorinatedalkenyl groups, such as alkenyl groups having one or more hydrogen atomsreplaced with one or more fluorine atoms.

Aryl groups include groups having one or more 5-, 6-, 7- or 8-memberaromatic rings, including heterocyclic aromatic rings. The termheteroaryl specifically refers to aryl groups having at least one 5-,6-, 7- or 8-member heterocyclic aromatic rings. Aryl groups can containone or more fused aromatic rings, including one or more fusedheteroaromatic rings, and/or a combination of one or more aromatic ringsand one or more nonaromatic rings that may be fused or linked viacovalent bonds. Heterocyclic aromatic rings can include one or more N,O, or S atoms in the ring. Heterocyclic aromatic rings can include thosewith one, two or three N atoms, those with one or two O atoms, and thosewith one or two S atoms, or combinations of one or two or three N, O orS atoms. Aryl groups are optionally substituted. Substituted aryl groupsinclude among others those that are substituted with alkyl or alkenylgroups, which groups in turn can be optionally substituted. Specificaryl groups include phenyl, biphenyl groups, pyrrolidinyl,imidazolidinyl, tetrahydrofuryl, tetrahydrothienyl, furyl, thienyl,pyridyl, quinolyl, isoquinolyl, pyridazinyl, pyrazinyl, indolyl,imidazolyl, oxazolyl, thiazolyl, pyrazolyl, pyridinyl, benzoxadiazolyl,benzothiadiazolyl, and naphthyl groups, all of which are optionallysubstituted. Substituted aryl groups include fully halogenated orsemihalogenated aryl groups, such as aryl groups having one or morehydrogens replaced with one or more fluorine atoms, chlorine atoms,bromine atoms and/or iodine atoms. Substituted aryl groups include fullyfluorinated or semifluorinated aryl groups, such as aryl groups havingone or more hydrogens replaced with one or more fluorine atoms. Arylgroups include, but are not limited to, aromatic group-containing orheterocylic aromatic group-containing groups corresponding to any one ofthe following: benzene, naphthalene, naphthoquinone, diphenylmethane,fluorene, anthracene, anthraquinone, phenanthrene, tetracene,tetracenedione, pyridine, quinoline, isoquinoline, indoles, isoindole,pyrrole, imidazole, oxazole, thiazole, pyrazole, pyrazine, pyrimidine,purine, benzimidazole, furans, benzofuran, dibenzofuran, carbazole,acridine, acridone, phenanthridine, thiophene, benzothiophene,dibenzothiophene, xanthene, xanthone, flavone, coumarin, azulene oranthracycline. As used herein, a group corresponding to the groupslisted above expressly includes an aromatic or heterocyclic aromaticgroup, including monovalent, divalent and polyvalent groups, of thearomatic and heterocyclic aromatic groups listed herein provided in acovalently bonded configuration in the compounds of the invention at anysuitable point of attachment. In embodiments, aryl groups containbetween 5 and 30 carbon atoms. In embodiments, aryl groups contain onearomatic or heteroaromatic six-member ring and one or more additionalfive- or six-member aromatic or heteroaromatic ring. In embodiments,aryl groups contain between five and eighteen carbon atoms in the rings.Aryl groups optionally have one or more aromatic rings or heterocyclicaromatic rings having one or more electron donating groups, electronwithdrawing groups and/or targeting ligands provided as substituents.

Arylalkyl groups are alkyl groups substituted with one or more arylgroups wherein the alkyl groups optionally carry additional substituentsand the aryl groups are optionally substituted. Specific alkylarylgroups are phenyl-substituted alkyl groups, e.g., phenylmethyl groups.Alkylaryl groups are alternatively described as aryl groups substitutedwith one or more alkyl groups wherein the alkyl groups optionally carryadditional substituents and the aryl groups are optionally substituted.Specific alkylaryl groups are alkyl-substituted phenyl groups such asmethylphenyl. Substituted arylalkyl groups include fully halogenated orsemihalogenated arylalkyl groups, such as arylalkyl groups having one ormore alkyl and/or aryl groups having one or more hydrogens replaced withone or more fluorine atoms, chlorine atoms, bromine atoms and/or iodineatoms.

As used herein, the terms “alkylene” and “alkylene group” are usedsynonymously and refer to a divalent group derived from an alkyl groupas defined herein. The invention includes compounds having one or morealkylene groups. Alkylene groups in some compounds function as attachingand/or spacer groups. Compounds of the invention may have substitutedand/or unsubstituted C₁-C₂₀ alkylene, C₁-C₁₀ alkylene and C₁-C₅ alkylenegroups.

As used herein, the terms “cycloalkylene” and “cycloalkylene group” areused synonymously and refer to a divalent group derived from acycloalkyl group as defined herein. The invention includes compoundshaving one or more cycloalkylene groups. Cycloalkyl groups in somecompounds function as attaching and/or spacer groups. Compounds of theinvention may have substituted and/or unsubstituted C₃-C₂₀cycloalkylene, C₃-C₁₀ cycloalkylene and C₃-C₅ cycloalkylene groups.

As used herein, the terms “arylene” and “arylene group” are usedsynonymously and refer to a divalent group derived from an aryl group asdefined herein. The invention includes compounds having one or morearylene groups. In an embodiment, an arylene is a divalent group derivedfrom an aryl group by removal of hydrogen atoms from two intra-ringcarbon atoms of an aromatic ring of the aryl group. Arylene groups insome compounds function as attaching and/or spacer groups. Arylenegroups in some compounds function as chromophore, fluorophore, aromaticantenna, dye and/or imaging groups. Compounds of the invention includesubstituted and/or unsubstituted C₃-C₃₀ arylene, C₃-C₂₀ arylene, C₃-C₁₀arylene and C₁-C₅ arylene groups.

As used herein, the terms “heteroarylene” and “heteroarylene group” areused synonymously and refer to a divalent group derived from aheteroaryl group as defined herein. The invention includes compoundshaving one or more heteroarylene groups. In an embodiment, aheteroarylene is a divalent group derived from a heteroaryl group byremoval of hydrogen atoms from two intra-ring carbon atoms or intra-ringnitrogen atoms of a heteroaromatic or aromatic ring of the heteroarylgroup. Heteroarylene groups in some compounds function as attachingand/or spacer groups. Heteroarylene groups in some compounds function aschromophore, aromatic antenna, fluorophore, dye and/or imaging groups.Compounds of the invention include substituted and/or unsubstitutedC₃-C₃₀ heteroarylene, C₃-C₂₀ heteroarylene, C₁-C₁₀ heteroarylene andC₃-C₅ heteroarylene groups.

As used herein, the terms “alkenylene” and “alkenylene group” are usedsynonymously and refer to a divalent group derived from an alkenyl groupas defined herein. The invention includes compounds having one or morealkenylene groups. Alkenylene groups in some compounds function asattaching and/or spacer groups. Compounds of the invention includesubstituted and/or unsubstituted C₂-C₂₀ alkenylene, C₂-C₁₀ alkenyleneand C₂-C₅ alkenylene groups.

As used herein, the terms “cylcoalkenylene” and “cylcoalkenylene group”are used synonymously and refer to a divalent group derived from acylcoalkenyl group as defined herein. The invention includes compoundshaving one or more cylcoalkenylene groups. Cycloalkenylene groups insome compounds function as attaching and/or spacer groups. Compounds ofthe invention include substituted and/or unsubstituted C₃-C₂₀cylcoalkenylene, C₃-C₁₀ cylcoalkenylene and C₃-C₅ cylcoalkenylenegroups.

As used herein, the terms “alkynylene” and “alkynylene group” are usedsynonymously and refer to a divalent group derived from an alkynyl groupas defined herein. The invention includes compounds having one or morealkynylene groups. Alkynylene groups in some compounds function asattaching and/or spacer groups. Compounds of the invention includesubstituted and/or unsubstituted C₂-C₂₀ alkynylene, C₂-C₁₀ alkynyleneand C₂-C₅ alkynylene groups.

As used herein, the term “halo” refers to a halogen group such as afluoro (—F), chloro (—Cl), bromo (—Br), iodo (—I) or astato (—At).

The term “heterocyclic” refers to ring structures containing at leastone other kind of atom, in addition to carbon, in the ring. Examples ofsuch heteroatoms include nitrogen, oxygen and sulfur. Heterocyclic ringsinclude heterocyclic alicyclic rings and heterocyclic aromatic rings.Examples of heterocyclic rings include, but are not limited to,pyrrolidinyl, piperidyl, imidazolidinyl, tetrahydrofuryl,tetrahydrothienyl, furyl, thienyl, pyridyl, quinolyl, isoquinolyl,pyridazinyl, pyrazinyl, indolyl, imidazolyl, oxazolyl, thiazolyl,pyrazolyl, pyridinyl, benzoxadiazolyl, benzothiadiazolyl, triazolyl andtetrazolyl groups. Atoms of heterocyclic rings can be bonded to a widerange of other atoms and functional groups, for example, provided assubstituents.

The term “carbocyclic” refers to ring structures containing only carbonatoms in the ring. Carbon atoms of carbocyclic rings can be bonded to awide range of other atoms and functional groups, for example, providedas substituents.

The term “alicyclic ring” refers to a ring, or plurality of fused rings,that is not an aromatic ring. Alicyclic rings include both carbocyclicand heterocyclic rings.

The term “aromatic ring” refers to a ring, or a plurality of fusedrings, that includes at least one aromatic ring group. The term aromaticring includes aromatic rings comprising carbon, hydrogen andheteroatoms. Aromatic ring includes carbocyclic and heterocyclicaromatic rings. Aromatic rings are components of aryl groups.

The term “fused ring” or “fused ring structure” refers to a plurality ofalicyclic and/or aromatic rings provided in a fused ring configuration,such as fused rings that share at least two intra ring carbon atomsand/or heteroatoms.

As used herein, the term “alkoxyalkyl” refers to a substituent of theformula alkyl-O-alkyl.

As used herein, the term “polyhydroxyalkyl” refers to a substituenthaving from 2 to 12 carbon atoms and from 2 to 5 hydroxyl groups, suchas the 2,3-dihydroxypropyl, 2,3,4-tri hydroxybutyl or2,3,4,5-tetrahydroxypentyl residue.

As used herein, the term “polyalkoxyalkyl” refers to a substituent ofthe formula alkyl-(alkoxy)n-alkoxy wherein n is an integer from 1 to 10,preferably 1 to 4, and more preferably for some embodiments 1 to 3.

Amino acids include glycine, alanine, valine, leucine, isoleucine,methionine, proline, phenylalanine, tryptophan, asparagine, glutamine,glycine, serine, threonine, serine, asparagine, glutamine, tyrosine,cysteine, lysine, arginine, histidine, aspartic acid, glutamic acid,selenocysteine and pyrrolysine. As used herein, reference to “a sidechain residue of a natural α-amino acid” specifically includes the sidechains of the above-referenced amino acids.

As to any of the groups described herein that contain one or moresubstituents, it is understood that such groups do not contain anysubstitution or substitution patterns which are sterically impracticaland/or synthetically non-feasible. In addition, the compounds of thisinvention include all stereochemical isomers arising from thesubstitution of these compounds. Optional substitution of alkyl groupsincludes substitution with one or more alkenyl groups, aryl groups orboth, wherein the alkenyl groups or aryl groups are optionallysubstituted. Optional substitution of alkenyl groups includessubstitution with one or more alkyl groups, aryl groups, or both,wherein the alkyl groups or aryl groups are optionally substituted.Optional substitution of aryl groups includes substitution of the arylring with one or more alkyl groups, alkenyl groups, or both, wherein thealkyl groups or alkenyl groups are optionally substituted.

Optional substituents for any alkyl, alkenyl and aryl group includessubstitution with one or more of the following substituents, amongothers:

halogen, including fluorine, chlorine, bromine or iodine;

pseudohalides, including —CN;

—COOR, where R is a hydrogen or an alkyl group or an aryl group and morespecifically where R is a methyl, ethyl, propyl, butyl, or phenyl groupall of which groups are optionally substituted;

—COR, where R is a hydrogen or an alkyl group or an aryl group and morespecifically where R is a methyl, ethyl, propyl, butyl, or phenyl groupall of which groups are optionally substituted;

—CON(R)₂, where each R, independently of each other R, is a hydrogen oran alkyl group or an aryl group and more specifically where R is amethyl, ethyl, propyl, butyl, or phenyl group all of which groups areoptionally substituted; and where R and R can form a ring which cancontain one or more double bonds and can contain one or more additionalcarbon atoms;

—OCON(R)₂, where each R, independently of each other R, is a hydrogen oran alkyl group or an aryl group and more specifically where R is amethyl, ethyl, propyl, butyl, or phenyl group all of which groups areoptionally substituted; and where R and R can form a ring which cancontain one or more double bonds and can contain one or more additionalcarbon atoms;

—N(R)₂, where each R, independently of each other R, is a hydrogen, oran alkyl group, or an acyl group or an aryl group and more specificallywhere R is a methyl, ethyl, propyl, butyl, phenyl or acetyl group, allof which are optionally substituted; and where R and R can form a ringthat can contain one or more double bonds and can contain one or moreadditional carbon atoms;

—SR, where R is hydrogen or an alkyl group or an aryl group and morespecifically where R is hydrogen, methyl, ethyl, propyl, butyl, or aphenyl group, which are optionally substituted;

—SO₂R, or —SOR, where R is an alkyl group or an aryl group and morespecifically where R is a methyl, ethyl, propyl, butyl, or phenyl group,all of which are optionally substituted;

—OCOOR, where R is an alkyl group or an aryl group;

—SO₂N(R)₂, where each R, independently of each other R, is a hydrogen,or an alkyl group, or an aryl group all of which are optionallysubstituted and wherein R and R can form a ring that can contain one ormore double bonds and can contain one or more additional carbon atoms;

—OR, where R is H, an alkyl group, an aryl group, or an acyl group allof which are optionally substituted. In a particular example R can be anacyl yielding —OCOR″, wherein R″ is a hydrogen or an alkyl group or anaryl group and more specifically where R″ is methyl, ethyl, propyl,butyl, or phenyl groups all of which groups are optionally substituted.

Specific substituted alkyl groups include haloalkyl groups, particularlytrihalomethyl groups and specifically trifluoromethyl groups. Specificsubstituted aryl groups include mono-, di-, tri, tetra- andpentahalo-substituted phenyl groups; mono-, di-, tri-, tetra-, penta-,hexa-, and hepta-halo-substituted naphthalene groups; 3- or4-halo-substituted phenyl groups, 3- or 4-alkyl-substituted phenylgroups, 3- or 4-alkoxy-substituted phenyl groups, 3- or4-RCO-substituted phenyl, 5- or 6-halo-substituted naphthalene groups.More specifically, substituted aryl groups include acetylphenyl groups,particularly 4-acetylphenyl groups; fluorophenyl groups, particularly3-fluorophenyl and 4-fluorophenyl groups; chlorophenyl groups,particularly 3-chlorophenyl and 4-chlorophenyl groups; methylphenylgroups, particularly 4-methylphenyl groups; and methoxyphenyl groups,particularly 4-methoxyphenyl groups.

As to any of the above groups that contain one or more substituents, itis understood that such groups do not contain any substitution orsubstitution patterns which are sterically impractical and/orsynthetically non-feasible. In addition, the compounds of this inventioninclude all stereochemical isomers arising from the substitution ofthese compounds.

The compounds of this invention can contain one or more chiral centers.Accordingly, this invention is intended to include racemic mixtures,diasteromers, enantiomers, tautomers and mixtures enriched in one ormore stereoisomer. The scope of the invention as described and claimedencompasses the racemic forms of the compounds as well as the individualenantiomers and non-racemic mixtures thereof.

DETAILED DESCRIPTION OF THE INVENTION

In general, the terms and phrases used herein have their art-recognizedmeaning, which can be found by reference to standard texts, journalreferences and contexts known to those skilled in the art. The followingdefinitions are provided to clarify their specific use in the context ofthe invention.

Reduction generally refers to a process involving the gain of one ormore electrons or a decrease in the oxidation state of an atom or groupof atoms in a molecule or ion. Reduction may refer to a reduction halfreaction in an oxidation-reduction reaction (i.e., Redox reaction).Reduction may involve reaction of a reactant undergoing reduction with areductant (also referred to as reducing agent), thereby resulting in areduction product having one or more atoms characterized by a loweroxidation state as compared to the reactant undergoing reduction. In anembodiment, for example, reduction of molecular nitrogen (N₂) results ingeneration of an ammonia (NH₃) or hydrazine (N₂H₄) reduction product.

“Reduction product” refers to a reaction product of a reductionreaction. A reduction product may be characterized by one or more atomshaving a lower oxidation state as compared to that of a reactantundergoing reduction. For example, molecular nitrogen (N₂) can bereduced to form the reduction product ammonia (NH₃) or hydrazine (N₂H₄).

“Metal complex” refers to a composition comprising one or more metalatoms or ions bound to one or more ligands. In an embodiment, metalcomplex refers to a coordination complex comprising one or moretransition metal atom, such as Fe or Co, bound to one or more phosphineligand, and optionally one or more other groups. The invention includesmetal complexes comprising a transition metal atom selected from thegroup consisting of Fe and Co and a phosphine ligand (L), and optionallya N₂ group. Metal complexes may be neutral or ionically charged, forexample, having a charge of +2, +1, −1 or −2.

“Catalyst” refers to a composition that increases the rate of one ormore chemical reactions of one or more reactants and is not consumed inthe chemical reaction. In an embodiment, for example, a catalyst lowersthe activation energy of a chemical reaction, thereby requiring lessenergy to achieve the transition state(s). Catalysts may participate inmultiple chemical transformations in a reaction sequence, therebyincreasing the rate of on overall transformation of reactants toproducts. In an embodiment, the invention provides catalysts forreduction of molecular nitrogen, for example, transition metal catalystscomprising an iron or cobalt complex.

This invention is further explained with the following embodiments,which are not intended to limit the scope of this invention.

FIG. 1A provides a schematic overview of a catalytic cycle for reductionof molecular nitrogen (N₂) to ammonia (NH₃) involving a transition metalcatalyst of the invention. In FIG. 1A, the symbol “Fe” is used as ashort hand representation of an iron coordination complex of theinvention comprising an iron atom and a phosphine ligand (L). Thus, inFIG. 1A, “Fe” is not merely depicting an iron atom but rather designatesan Fe complex of the invention. As shown in FIG. 1, the dinitrogenadduct (Fe—N₂)⁻ under goes protonation and reduction reactions togenerate various hydrogenated metal complex intermediates prior torelease of a NH₃ reduction product.

FIG. 1B provides a schematic overview of a catalytic cycle for reductionof molecular nitrogen (N₂) to ammonia (NH₃) involving the specific metalcomplex (TPB)Fe(N₂)⁻, which comprises an iron atom bound to atris(phosphinoaryl)borane ligand and a N₂ group. As shown in FIG. 1B,the (TPB)Fe(N₂) catalyst participates in protonation and reductionreactions in the presence of 1 atmosphere of N₂ resulting in generationof a NH₃ reduction product.

EXAMPLE 1 Catalytic Conversion of Nitrogen to Ammonia by a Molecular FeModel Complex

Nitrogen reduction to NH₃ is a requisite transformation for life.¹ Whileit is widely appreciated that the Fe-rich cofactors of nitrogenaseenzymes facilitate this transformation,^(2,3,4,5) how they do so remainspoorly understood. A central element of debate has been the site(s) ofnitrogen coordination and reduction.^(6,7) The synthetic inorganiccommunity placed an early emphasis on Mo⁸ because Mo was thought to bean essential element of nitrogenases,³ and because pioneering work byChatt and his coworkers established that well-defined Mo model complexescould mediate the stoichiometric conversion of coordinated N₂ to NH₃.⁹Indeed, such a transformation has now been validated in a catalyticfashion by two well-defined molecular systems that feature Mocentres.^(10,11) It is now thought that Fe is the only transition metalessential to all nitrogenases,³ and recent biochemical and spectroscopicdata has implicated Fe instead of Mo as the site of N₂ binding in theFeMo-cofactor.¹² These observations motivate a search for functional Fecatalysts. In this Example, we disclose a tris(phosphine)borane (TPB)supported Fe complex that catalyzes the reduction of N₂ to NH₃ undermild conditions, wherein >40% of the H⁺/e⁻ equivalents are delivered toN₂. These results intimate that a single Fe site is capable ofstabilizing the various N_(x)H_(y) intermediates generated en route tocatalytic NH₃ formation. Geometric tunability at Fe imparted by aflexible Fe—B interaction in the model system appears to be importantfor efficient catalysis.^(13,14,15) The results also support theinterpretation that the interstitial light C-atom recently assigned inthe nitrogenase cofactor may play a similar role,^(16,17) potentiallyenabling a single Fe site to mediate the enzymatic catalysis via aflexible Fe—C interaction.¹⁸

Nitrogen is fixed on a staggering scale by the industrial Haber-Boschprocess using a solid-state Fe catalyst at very high temperatures andpressures, and in nature by nitrogenase enzymes under ambientconditions.¹ These enzymes feature active site cofactors rich in S andFe (FeFe-cofactor), and can additionally contain a Mo (FeMo-cofactor;FIG. 2) or V (FeV-cofactor) site.^(2,3)

The intimate mechanism of biological nitrogen fixation is a topic thathas fascinated chemists, biochemists, and biologists alike.^(4,5,6,7)Synthetic chemists have searched for decades for well-defined complexesthat can catalyze N₂ reduction to NH₃.^(19,20,21,22) Chatt's early workwith low-valent Mo complexes provided a mechanistic outline forapproaching this problem now commonly called the “Chatt” or “distal”mechanism, wherein a terminal nitride intermediate is generated uponliberation of the first NH₃ equivalent (FIG. 2, bottom).⁹ Othermechanisms, including an “alternating” scenario (FIG. 2, top), have alsoreceived attention.⁶ To date, even modest catalysis of NH₃ productiondirectly from N₂ by a well-defined model complex is still limited to theoriginal tri(amido)amine Mo systems of Schrock and coworkers, and themore recently discovered phosphine-pincer Mo system of Nishibayashi andcoworkers.^(10,11) Earlier work by Pickett had established theelectrochemical feasibility of an NH₃ production cycle with W.¹⁹

Synthetic efforts to establish whether one or more Fe sites can catalyzeN₂ reduction to NH₃ in a well-defined model complex have progressed moreslowly. For example, previous Fe—N₂ model complexes have not affordedmore than ca. 10% of NH₃ per Fe equivalent upon treatment with protonsources.^(7,23,24) Despite significant advances,²⁴ which have mostrecently included reductive N₂ cleavage at iron²² and the catalyticreductive silylation of N₂ facilitated by unknown Fe species derivedfrom Fe precursors such as ferrocene and iron pentacarbonyl,²⁵ thedelivery of protons and electrons to N₂ to catalytically generate NH₃ ata synthetic Fe complex has remained an unsolved challenge. Here we showthat a mononuclear Fe complex, [(TPB)Fe(N₂)][Na(12-crown-4)₂](TPB=tris(phosphine)borane; see FIG. 3A)^(,13,14) can catalyze thereduction of N₂ to NH₃ by protons and electrons in solution at −78° C.under one atmosphere of N₂.

The Fe center of the “(TPB)Fe” fragment readily binds dinitrogen asevidenced by the featured 5-coordinate complex[(TPB)Fe(N₂)][Na(12-crown-4)₂] and the neutral N₂ adduct, (TPB)Fe(N₂).¹⁴The same scaffold also accommodates a variety of other nitrogenousligands relevant to NH₃ generation, including terminally bonded NH₂,NH₃, and N₂H₄ ligands.¹⁵ Both the substrate-free complex,[(TPB)Fe][BAr^(F) ₄] (where [BAr^(F) ₄]⁻ represents the weaklycoordinating anion [B(3,5-(CF₃)₂—C₆H₃)₄]⁻),¹⁵ and (TPB)Fe(N₂) may bereduced to [(TPB)Fe(N₂)][Na(12-crown-4)₂] by Na/Hg reduction under 1 atmN₂ followed by the addition of two equivalents of 12-crown-4 toencapsulate the sodium cation (FIG. 3A). Model reactions with silylelectrophiles have also shown that the β-N of the coordinated N₂ ligandcan be mono- or difunctionalized with concomitant lengthening of theFe—B distance.¹³ Furthermore, starting from (TPB)Fe(NH₂), a reductiveprotonation sequence has been established (FIG. 3B) that liberates NH₃and affords (TPB)Fe(N₂).¹⁵ This reaction sequence demonstrates thesynthetic viability of reductive release of NH₃ from an Fe—NH₂intermediate with simultaneous uptake of N₂. In sum, the rich reactionchemistry of the (TPB)Fe scaffold with nitrogenous ligands supports thepossibility that it might catalyze N₂ fixation.

The addition of excess acid to [(TPB)Fe(N₂)][Na(12-crown-4)₂] at −78° C.was investigated. When [(TPB)Fe(N₂)][Na(12-crown-4)₂] was dissolved inTHF, cooled to −78° C., and exposed to six equivalents of H⁺ in the formof HBAr^(F) ₄.2 Et₂O, a previously unobserved yellow solution resultedthat, upon warming followed by addition of proton sponge(1,8-bis(dimethylamino)naphthalene), was shown by ¹H NMR analysis tocontain the complex [(TPB)Fe(NH₃)][BAr^(F) ₄] (ca. 30-35% of the totalFe),¹⁵ along with resonances consistent with [(TPB)Fe][BAr^(F) ₄] (ca.40-45% of the total Fe) and two other minor and as yet unidentifiedparamagnetic (TPB)Fe-species (see Sl). An independent EPR study of thislow temperature protonation reaction in 2-methyltetrahydrofuran revealeda new rhombic S=½ signal (see Sl) that we speculate may be(TPB)Fe(═N—NH₂)⁺ or an alternative structural isomer such as(TPB)Fe(NH═NH)⁺. Spin quantification of this species shows it torepresent >85% of the Fe species in solution, and its rhombic EPRspectrum is highly similar to the rhombic EPR signature that is obtainedupon oxidation of (TPB)Fe═N(p-tolyl) to generate (TPB)Fe═N(p-tolyl)⁺(Sl). Subsequent low temperature reduction of a similarly preparedmixture regenerates [(TPB)FeN₂][Na(12-crown-4)₂], as determined by IRspectroscopy, suggesting the possibility of cycling protonation andreduction with this Fe system.

To explore the possibility of N₂ reduction catalysis using this (TPB)Fesystem, we canvassed several reductants (e.g., Na[naphthalenide],Mg(THF)₃(anthracene), Na/Hg, KC₈) and acids (e.g., HCl,trifluoromethanesulfonic acid, HBAr^(F) ₄.2 Et₂O) in a variety ofsolvents and solvent mixtures (e.g., tetrahydrofuran, dimethoxyethane,diethyl ether, toluene). When carried out at −78° C. numerous reactionconditions showed unusually high yields of NH₃ relative to the number ofFe equivalents in the reaction vessel, and the combination of KC₈,HBAr^(F) ₄.2 Et₂O, and Et₂O solvent enabled the catalytic generation ofNH₃.

In a representative catalytic run, red [(TPB)Fe(N₂)][Na(12-crown-4)₂]was suspended in diethyl ether in a reaction vessel at −78° C., followedby the sequential addition of excess acid and then excess reductant.Ammonia analysis followed the indophenol protocol (see Methods and SI)and the independent identification of ammonium salts by ¹H NMRspectroscopy in DMSO by comparison with an authentic sample of [NH₄][Cl](FIG. 4A). An experiment performed using the[(TPB)Fe(¹⁴N₂)][Na(12-crown-4)₂] catalyst under an ¹⁵N₂ atmosphere,followed by ¹H NMR analysis (FIG. 4A) of the volatile reaction products,confirmed the production of [¹⁵NH₄][Cl], as expected, with only trace[¹⁴NH₄][Cl]. This overall procedure has been repeated many times, andTable 1 includes data averaged from 16 independent runs (entry 1)wherein yields were, on average, 7.0 equiv NH₃ per Fe equiv. Using 7.0equiv NH₃ as the product stoichiometry, 44% of the added protons arereliably delivered to N₂ to produce NH₃. Individual runs have in ourhands reached a maximum of 8.5 equiv NH₃ per Fe equiv under thesestandard conditions. [(TPB)Fe][BAr^(F) ₄] is also an effective catalystand afforded 6.2±0.7 equiv NH₃ per added Fe equivalent (Table 1, entry2). For comparison, the Mo systems of Schrock and Nishibayashi haveafforded between 7-12 equiv NH₃ per Mo equiv.^(10,11) The current Fesystem appears to be active at a low temperature (−78° C.) but benefitsfrom a strong reductant (KC₈). Options for carrying out the process athigher temperatures may include circumventing generation of the(TPB)Fe(N₂)⁻ anion during catalysis.

Table 1 lists several sets of conditions (entries 10-15) other than thestandard conditions described above that were canvassed. Several ofthese alternative conditions showed NH₃ generation, though not incatalytic or in high yields. N₂H₄ was not detected (Sl) as an additionalproduct when using the standard catalytic protocol for NH₃ generationwith [(TPB)Fe(N₂)][Na(12-crown-4)₂] (Sl). If two equivalents N₂H₄ (perFe) are added to [(TPB)Fe(N₂)][Na(12-crown-4)₂] in diethyl ether,followed by subjecting the mixture to the standard catalytic conditionsand work-up, only trace N₂H₄ remains (Sl). This result supports theunderstanding that if N₂H₄ is generated as an intermediate duringcatalysis it would not likely be detectable upon work-up and analysis.Worth noting is that HBAr^(F) ₄.2 Et₂O and KC₈ reacts in the absence ofan Fe precursor, under the standard catalytic conditions at −78° C., togenerate H₂ but not NH₃ (>75% yield of H₂ after 40 minutes). That H₂generation is kinetically feasible without the addition of an Feprecursor, and yet NH₃ can nonetheless be generated upon the addition of[(TPB)Fe(N₂)][Na(12-crown-4)₂] or [(TPB)Fe][BAr^(F) ₄], underscores thefacility by which this Fe system mediates overall H-atom delivery to N₂.

To further explore whether a (TPB)Fe containing precursor is needed tofacilitate the overall catalysis, beyond the stoichiometric modelreactions summarized above, we canvassed several Fe complexes underanalogous conditions. Of most interest is the complex [(SiP^(iPr)₃)Fe(N₂)][Na(12-crown-4)₂], which is isostructural to[(TPB)Fe(N₂)][Na(12-crown-4)₂] but replaces the B atom of TPB by a Siatom.²⁶ A central difference between (TPB)Fe and (SiP^(iPr) ₃)Fecomplexes is the far great flexibility of the Fe—B versus the Fe—Si bondthat is positioned trans to the apical ligand.^(13,14,15,26) While someNH₃ generation was observed for [(SiP^(iPr) ₃)Fe(N₂)][Na(12-crown-4)₂]when subjected to the standard catalytic reaction conditions describedabove, sub-stoichiometric yields of NH₃ relative to Fe were obtained(0.7±0.5 equiv NH₃ per Fe equiv; entry 3). We also conducted additionalcontrol experiments under the standard catalytic conditions withFeCl₂.1.5 THF, FeCl₃, Cp₂Fe,²⁵ and Fe(CO)₅ ²⁵ (entries 5-8) and foundthat only trace amounts of NH₃ (<0.2 equiv in all cases on average; 4runs) were produced by these Fe precursors (Sl).²⁷ The knownphosphine-supported Fe(0)-N₂ complex Fe(depe)₂(N₂)²⁸ was also subjectedto the standard conditions and afforded sub-stoichiometric yields of NH₃per Fe equivalent.

In separate work, the addition of an atmosphere of H₂ to (TPB)Fe(N₂) wasshown to generate (TPB)(μ-H)Fe(N₂)(H) as a stable product (FIG. 4C).²⁹We hence suspect that catalyst poisoning might occur in part via theformation of (TPB)(μ-H)Fe(N₂)(H) under the catalytic reactionconditions. In accord with this idea, when[(TPB)Fe(N₂)][Na(12-crown-4)₂] was exposed to 10 equiv HBAr^(F) ₄.2 Et₂Oand 12 equiv KC₈ at low temperature, IR and ³¹P NMR analysis of theresulting mixture showed the presence of (TPB)(μ-H)Fe(N₂)(H) via itssignature spectroscopic features (30% of total Fe by ³¹P NMRintegration; Sl).²⁹ (TPB)(μ-H)Fe(N₂)(H) is stable for short periods toboth HBAr^(F) ₄.2 Et₂O and also KC₈ in Et₂O at room temperature, andwhen subjected to the standard catalytic conditions for NH₃ productionliberates only 0.5±0.1 equiv NH₃ per Fe equiv (Table 1 entry 4).

The general absence of a functional, catalytic Fe model system over thepast few decades has often led to an emphasis on Mo³⁰ as a plausiblesite of N₂ uptake and reduction at the most widely studiedFeMo-cofactor. While this may yet prove to be true, recent spectroscopicand biochemical evidence has sharpened the focus on an Fe center as theN₂ binding site.¹² The results provided in this Example establish thatit is possible to catalyze the conversion of N₂ to NH₃ by protons andelectrons using a well-defined mononuclear Fe—N₂ complex, and suggeststhe possibility that a single Fe-binding site of the cofactor could inprinciple mediate N₂ reduction catalysis.¹⁸ To achieve this catalyticbehavior, geometric flexibility at the Fe—N₂ binding site is beneficialin some embodiments as it would stabilize N_(x)H_(y) intermediates ofdifferent electronic demands. Such geometric and redox flexibility,under the local three-fold symmetry presented by an Fe center, its threeneighboring sulfides, and the interstitial light atom of theFeMo-co,^(16,17) may at least in part be achieved by attributing ahemi-labile role to the interstitial C-atom (FIG. 2). Such a role couldserve to expose an initial Fe—N₂ binding site by Fe—C elongation.Subsequent modulation of the Fe—C interaction and hence the local Fegeometry as a function of the N₂ reduction state would enable the Fecenter to stabilize the various N_(x)H_(y) intermediates along a pathwayto NH₃. This approach is rooted in the functional (TPB)Fe catalysisdescribed herein, along with the types of (TPB)Fe complexes andstoichiometric transformations described for this scaffold.^(13,14,15)

While all of the model complexes relevant to the (TPB)Fe—N_(x)H_(y)system are mononuclear, there is a possibility of bimolecular reactionintermediates. The N₂ reduction catalysis may proceed along a distalpathway via a terminal nitride intermediate, such as (TPB)Fe(N) or(TPB)Fe(N)⁺, via intermediates along an alternating pathway, such as(TPB)Fe—NH—NH₂ or (TPB)Fe—NH₂—NH₂ ⁺, or via some hybrid pathway. Theassigned (TPB)Fe═N—NH₂ ⁺ species that can be observed by EPRspectroscopy provides us a useful starting point for addressing thisissue. In light of the identification of C as the interstitial lightatom of the cofactor, it is also of interest to develop and comparesynthetic model systems that feature a C-atom in the ligand backboneinstead of a B-atom.

TABLE 1 Catalytic ammonia generation from N2 mediated by Fe precursors.Using standard catalytic conditions as described in the text Entry Feprecursor NH₃ equiv/Fe^(a,b,c) 1 [(TPB)Fe(N₂)][Na(12-crown-4)₂] 7.0 ±1^(d) 2 [(TPB)Fe][BAr^(F) ₄] 6.2 3 [(SiP^(iPr)₃)Fe(N₂)][Na(12-crown-4)₂] 0.7 4 (TPB)(μ-H)Fe(N₂)(H) 0.5 5 FeCl₂ · 1.5THF <0.1 6 FeCl₃ <0.1 7 Cp₂Fe <0.2 8 Fe(CO)₅ <0.1 9 None <0.1 Variationson standard conditions using [(TPB)FeN₂][Na(12-crown-4)₂] EntryVariation NH₃ equiv/Fe^(a,b,c) 10 HOTf as acid^(e) 0.4 11[Lutidinium][BAr^(F) ₄] as acid <0.1 12 HCl as acid <0.1 13 Cp*₂Co asreductant 0.6 14 Cp*₂Cr as reductant <0.2 15 K metal as reductant 0.4^(a)NH₃ was collected by vacuum transfer of the reaction volatiles intoHCl in diethyl ether. A dimethoxyethane solution of [Na][O^(t)Bu] (20equiv relative to Fe) was added to the reaction vessel residue, followedby an additional vacuum transfer, to ensure complete liberation of allNH₃. The [NH₄][Cl] precipitate formed in the acidic Et₂O collectionvessel was reconstituted in deionized H₂O, from which an aliquot wastaken for indophenol quantification. Analysis of the [NH₄][Cl] formed by¹H NMR spectroscopy in DMSO, compared with an authentic sample, providedindependent confirmation of NH₃ generation. ^(b)Data for individualexperimental runs, and additional runs using potential precatalysts thatare not presented in this table, are provided in the SI. ^(c)Unlessnoted otherwise, all yields are reported as an average of 4 runs.^(d)Average of 16 runs. ^(e)HOTf = trifluoromethanesulfonic acid.Methods Summary

General considerations. All complexes and reagents were preparedaccording to literature procedures referenced herein unless otherwisenoted. Manipulations were carried out under an N₂ atmosphere utilizingstandard dry glove-box or Schlenk-line techniques. All solvents usedwere deoxygenated and dried by an argon sparge followed by passagethrough an activated alumina column. Spectroscopy. NMR measurements wereobtained on Varian 300 MHz spectrometers. Deuterated solvents for thesemeasurements were obtained from Cambridge Isotope Laboratories and weredried and degassed prior to use. All ¹H NMR spectra were referenced toresidual solvent peaks. EPR X-band spectra were obtained on a Bruker EMXspectrometer with the aid of the Bruker Win-EPR software suite version3.0. The EPR spectrometer was equipped with a rectangular cavity thatoperated in the TE₁₀₂ mode. Temperature control was achieved with aliquid-N₂-filled quartz-dewar in which the sample was submerged duringdata collection. UV-Vis spectra were acquired on a Cary 50 spectrometerfrom 1100 nm to 200 nm in the fast scan mode. IR spectra were obtainedvia KBr pellets on a Bio-Rad Excalibur FTS 3000 spectrometer usingVarian Resolutions Pro software set at 4 cm⁻¹ resolution.

Catalysis and Ammonia Collection and Quantification. The standardcatalysis protocol involved the addition first of acid, followed byreductant, to a suspension of the precatalyst in diethyl ether at −78°C. in a closed vessel under 1 atm N⁻². Ammonia produced during eachcatalytic run was collected by vacuum transfer of the reaction volatilesonto anhydrous HCl in diethyl ether. The resulting slurry was dried andextracted into water and aliquots were then tested for the presence ofammonia via the indophenol method.

Methods

General Considerations. [(TPB)Fe(N₂)][Na(12-crown-4)₂],¹⁴[(TPB)Fe][BAr^(F) ₄],¹⁵ (TPB)(μ-H)Fe(H)(N₂),²⁹ [Lutidinium][BAr^(F)₄],³¹ HBAr^(F) ₄.2 Et₂O,³² [(SiP^(iPr) ₃)Fe(N₂)][Na(12-crown-4)₂],²⁶FeCl₂.(THF)_(1.5),³³ KC₈,³⁴ [(TPB)Fe(NH₃)][BAr^(F) ₄],¹⁵[(TPB)Fe(N₂H₄)][BAr^(F) ₄],¹⁵ and Fe(depe)₂N₂ ²⁸ were prepared accordingto literature procedures. Note: [Lutidinium]=2,6-dimethylpyridinium;[BAr^(F) ₄]=[B(3,5-(CF₃)₂—C₆H₃)₄]⁻. Labeled ¹⁵N₂ (98% purity) wasobtained from Cambridge Isotope Laboratories. Solvents used forcatalytic runs were additionally stirred for more than 2 hours over Na/Kalloy and then filtered prior to use, in addition to standard sparging(Ar gas) and passage through an activated alumina column.

Ammonia Quantification. A Schlenk tube was charged with HCl (3 mL of a2.0 M solution in Et₂O, 6 mmol). Reaction mixtures were vacuumtransferred into this collection flask. Residual solid in the reactionvessel was treated with a solution of [Na][O-t-Bu] (40 mg, 0.4 mmol) in1,2-dimethoxyethane (1 mL) and sealed. The resulting suspension wasallowed to stir for 10 minutes before all volatiles were again vacuumtransferred into the collection flask. After completion of the vacuumtransfer, the flask was sealed and warmed to room temperature. Solventwas removed in vacuo and the remaining residue was dissolved in H₂O (1mL). An aliquot of this solution (20 or 40 μL) was then analyzed for thepresence of NH₃ (trapped as [NH₄][Cl]) via the indophenol method.³⁵Quantification was performed with UV-Vis spectroscopy by analyzingabsorbance at 635 nm. The tables shown indicate the raw data for theruns. Runs with small absorbance levels (<0.02 absorbance units) sufferfrom a large degree of error due to a small signal-to-noise ratio.Catalytic runs that used a 40 μL aliquot are denoted with an asterisk,accounting for larger relative absorbances.

Standard Catalytic Protocol. [(TPB)Fe(N₂)][Na(12-crown-4)₂] (2 mg, 0.002mmol) was suspended in Et₂O (0.5 mL) in a 20 mL scintillation vialequipped with a stir bar. This suspension was vigorously stirred andcooled to −78° C. in a cold well inside of the glove box. A similarlycooled solution of HBAr^(F) ₄.2 Et₂O (93 mg, 0.092 mmol) in Et₂O (1.5mL) was added to the suspension in one portion with rapid stirring. Anyremaining acid was dissolved in cold Et₂O (0.25 mL) and addedsubsequently. The reaction mixture turned light yellow-orange andhomogeneous upon addition of acid and the resulting solution was allowedto stir for 5 minutes before being transferred into a pre-cooled Schlenktube equipped with a stirbar. The original reaction vial was washed withcold Et₂O (0.25 mL) and was subsequently transferred to the Schlenktube. Solid KC₈ (15 mg, 0.100 mmol) was suspended in cold Et₂O (0.75 mL)and added dropwise to the rapidly stirred solution in the Schlenk tubeand was then tightly sealed. The reaction was allowed to stir for 40minutes at −78° C. before being warmed to room temperature and stirredfor 15 minutes.

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Unless otherwise noted, all compounds were purchased from commercialsources and used without further purification.[(TPB)Fe(N₂)][Na(12-crown-4)₂],¹⁴ [(TPB)Fe][BAr^(F) ₄],¹⁵(TPB)(μ-H)Fe(H)(N₂),²⁹ [Lutidinium][BAr^(F) ₄],³¹ HBAr^(F) ₄.2 Et₂O,³²[(SiP^(iPr) ₃)Fe(N₂)][Na(12-crown-4)₂],²⁶ FeCl₂.1.5 THF,³³ KC₈,³⁴[(TPB)Fe(NH₃)][BAr^(F) ₄],¹⁵ [(TPB)Fe(N₂H₄)][BAr^(F) ₄],¹⁵(TPB)Fe≡N(p-C₆H₄OMe),¹⁴ and Fe(depe)₂N₂ ³⁵ were prepared according toliterature procedures ([Lutidinium]=2,6-dimethylpyridinium, [BAr^(F)₄]=[B(3,5-(CF₃)₂—C₆H₃)₄B]⁻). All manipulations were carried out under anN₂ atmosphere utilizing standard glovebox or Schlenk techniques.Solvents were dried and de-oxygenated by an argon sparge followed bypassage through an activated alumina column purchased from S.G. WatersCompany. Labeled ¹⁵N₂ (98% purity) was obtained from Cambridge IsotopeLaboratories. Solvents for catalytic runs were additionally stirred formore than 2 hours over Na/K alloy then filtered prior to use.

IR Spectroscopy

IR spectra were obtained via KBr pellets on a Bio-Rad Excalibur FTS 3000spectrometer using Varian Resolutions Pro software set at 4 cm⁻¹resolution.

NMR Spectroscopy

NMR measurements were obtained on Varian 300 MHz or 500 MHzspectrometers. Deuterated solvents for these measurements were obtainedfrom Cambridge Isotope Laboratories and were dried and degassed prior touse. All ¹H NMR spectra were referenced to residual solvent peaks.

EPR Spectroscopy

EPR X-band spectra were obtained on a Bruker EMX spectrometer with theaid of Bruker Win-EPR software suite version 3.0. The spectrometer wasequipped with a rectangular cavity which operated in the TE₁₀₂ mode.Temperature control was achieved with a liquid N₂ filled quartz dewar inwhich the sample was submerged during data collection.

UV-Visible Spectroscopy

UV-Visible spectra were taken on a Cary 50 spectrometer from 1100 nm to200 nm in the fast scan mode. Samples were prepared in a 1 cm pathlength quartz cuvette. All samples had a blank sample backgroundsubtraction applied.

Standard Catalytic Protocol

[(TPB)Fe(N₂)][Na(12-crown-4)₂] (2 mg, 0.002 mmol) was suspended in Et₂O(0.5 mL) in a 20 mL scintillation vial equipped with a stir bar. Thissuspension was vigorously stirred and cooled to −78° C. in a cold wellinside of the glove box. A similarly cooled solution of HBAr^(F) ₄.2Et₂O (93 mg, 0.092 mmol) in Et₂O (1.5 mL) was added to the suspension inone portion with rapid stirring. Any remaining acid was dissolved incold Et₂O (0.25 mL) and added subsequently. The reaction mixture turnedlight yellow-orange and homogeneous upon addition of acid and theresulting solution was allowed to stir for 5 minutes before beingtransferred into a pre-cooled Schlenk tube equipped with a stir bar. Theoriginal reaction vial was washed with cold Et₂O (0.25 mL) which wassubsequently transferred to the Schlenk tube. Solid KC₈ (15 mg, 0.100mmol) was suspended in cold Et₂O (0.75 mL) and added dropwise to therapidly stirred solution in the Schlenk tube which was then tightlysealed. The reaction was allowed to stir for 40 minutes at −78° C.before being warmed to room temperature and stirred for 15 minutes.

Ammonia Quantification

A Schlenk tube was charged with HCl (3 mL of a 2.0 M solution in Et₂O, 6mmol). Reaction mixtures were vacuum transferred into this collectionflask. Residual solid in the reaction vessel was treated with a solutionof [Na][O-t-Bu] (40 mg, 0.4 mmol) in 1,2-dimethoxyethane (1 mL) andsealed. The resulting suspension was allowed to stir for 10 minutesbefore all volatiles were again vacuum transferred into the collectionflask. After completion of the vacuum transfer, the flask was sealed andwarmed to room temperature. Solvent was removed in vacuo and theremaining residue was dissolved in H₂O (1 mL). An aliquot of thissolution (20 or 40 μL) was then analyzed for the presence of NH₃(trapped as [NH₄][Cl]) via the indophenol method.³⁶ Quantification wasperformed with UV-Visible spectroscopy by analyzing the absorbance at635 nm. The tables shown below list the raw data for the runs. Runs withsmall absorbance levels (<0.02 absorbance units) suffer from a largedegree of error due to a small signal-to-noise ratio. Catalytic runsthat used a 40 μL aliquot are denoted with an asterisk, accounting forlarger relative absorbances.

TABLE 2 N₂ reduction catalysis absorption data using[(TPB)Fe(N₂)][Na(12-crown-4)₂]. Run Absorbance Equiv NH₃/Fe % Yieldbased on H⁺ A* 1.095 6.52 40.7 B* 1.150 6.84 42.7 C* 0.724 4.30 26.9 D*1.105 6.58 41.1 E* 1.165 6.93 43.3 F* 1.339 7.97 49.8 G* 1.050 6.25 39.1H* 1.428 8.49 53.1 I* 1.418 8.44 52.7 J* 1.008 6.00 37.5 L 0.608 7.2445.2 M 0.579 6.89 43.1 N 0.640 7.62 47.6 O 0.592 7.05 44.1 P 0.616 7.3345.8Catalytic Protocol Under ¹⁵N₂

[(TPB)Fe(N₂)][Na(12-crown-4)₂] (4 mg, 0.004 mmol) was suspended in Et₂O(3 mL) in a 25 mL three neck flask (ground-glass, 14/20) equipped with astir bar. The flask was then equipped with a stopcock adaptor in thecentral opening, a solid addition arm containing HBAr^(F) ₄.2 Et₂O (188mg, 0.186 mmol) in one of the side openings, and an additional solidaddition arm containing KC₈ (37 mg, 0.274 mmol) in the final opening.The apparatus was sealed, brought out of the glovebox, and connected toa high-vacuum manifold. The solution was degassed via fourfreeze-pump-thaw cycles and then allowed to thaw to −78° C. withstirring. The flask was backfilled with 1 atm ¹⁵N₂. At this point theacid was added to the solution in one portion via the solid additionarm. The resulting solution was allowed to stir for 5 minutes before KC₈was added via the other solid addition arm resulting in a darksuspension. This suspension was allowed to stir for 40 minutes at −78°C. and then an additional 10 minutes at room temperature prior to thestandard work-up. The presence of [¹⁵NH₄][Cl] was verified by ¹H NMRspectroscopy (FIG. 8). The yield of NH₃ was 3.64 equiv NH₃/Fe asmeasured using the indophenol method. The NH₃ yield, while still showingcatalysis, was lower than the average obtained by the standard protocol,presumably due to differences associated with adding the HBAr^(F) ₄.2Et₂O and KC₈ solids via the solid addition arms.

Runs with [(TPB)Fe][BAr^(F) ₄] as Precursor

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The precursor used was [(TPB)Fe][BAr^(F) ₄] (2.3mg, 0.002 mmol) which is a dark orange solid. Note that[(TPB)Fe][BAr^(F) ₄] was soluble in Et₂O and formed a yellow solution.No substantial color change was observed upon addition of acid.

TABLE 3 N₂ reduction catalysis absorption data using [(TPB)Fe][BAr^(F)₄]. Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ A* 1.169 6.96 43.5B* 1.000 5.95 37.2 C* 0.911 5.42 33.9 D* 1.117 6.65 41.6Runs with [(SiP^(iPr) ₃)Fe(N₂)][Na(12-crown-4)₂] as Precursor

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The precursor used was [(SiP^(iPr)₃)Fe(N₂)][Na(12-crown-4)₂] (2 mg, 0.002 mmol) which is a dark purplesolid.

TABLE 4 Attempted N₂ reduction catalysis absorption data using[(SiP^(iPr) ₃)Fe(N₂)][Na(12-crown-4)₂]. Run Absorbance Equiv NH₃/Fe %Yield based on H⁺ A* 0.203 1.21 7.5 B* 0.059 0.35 2.1 C* 0.064 0.38 2.3D* 0.183 1.09 6.8Runs with (TPB)(μ-H)Fe(H)(N₂) as Precursor

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The precursor used was (TPB)(μ-H)Fe(H)(N₂) (1.3mg, 0.002 mmol) which is a yellow solid. Note that (TPB)(μ-H)Fe(H)(N₂)was insoluble in Et₂O and did not dissolve upon addition of acid. Assuch, the resulting mixture was a suspension through the remainingmanipulations.

TABLE 5 Attempted N₂ reduction catalysis absorption data using(TPB)(μ-H)Fe(H)(N₂). Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ A*0.084 0.50 3.1 B* 0.072 0.43 2.7 C 0.035 0.42 2.6 D 0.055 0.65 4.1Runs with FeCl₂.1.5 THF as Precursor

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The precursor used was FeCl₂.1.5 THF (0.5 mg,0.002 mmol) which is an off white powder. Note that FeCl₂.1.5 THF didnot dissolve upon addition of acid. As such, the resulting mixture was asuspension through the remaining manipulations.

TABLE 6 Attempted N₂ reduction catalysis absorption data using FeCl₂ ·1.5 THF. Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ A 0.002 0.020.1 B 0.011 0.13 0.8 C 0.005 0.06 0.4 D 0.007 0.08 0.5Runs with FeCl₃ as Precursor

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The precursor used was FeCl₃ (0.3 mg, 0.002mmol) which is a dark solid. Note that FeCl₃ was soluble in Et₂O andformed a yellow solution. No substantial color change was observed uponaddition of acid.

TABLE 7 Attempted N₂ reduction catalysis absorption data using FeCl₃.Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ A −0.0021 0 0 B −0.00020 0 C 0.0002 0.002 0.01 D 0.0010 0.01 0.06Runs with Fe(CO)₅ as Precursor

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The precursor used was Fe(CO)₅ (0.35 mg, 0.002mmol) which is a pale yellow liquid. Note that Fe(CO)₅ was soluble inEt₂O and formed a colorless solution. No substantial color change wasobserved upon addition of acid.

TABLE 8 Attempted N₂ reduction catalysis absorption data using Fe(CO)₅.Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ A 0.016 0.19 1.2 B 0.0030.04 0.2 C 0.004 0.05 0.3 D 0.006 0.07 0.4Runs with FeCp₂ as Precursor

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The precursor used was FeCp₂ (0.35 mg, 0.002mmol) which is an orange solid. Note that FeCp₂ was soluble in Et₂O andformed a yellow solution. No substantial color change was observed uponaddition of acid.

TABLE 9 Attempted N₂ reduction catalysis absorption data using FeCp₂.Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ A 0.007 0.08 0.5 B 0.0180.21 1.3 C 0.027 0.32 2.0 D 0.015 0.18 1.1Runs without an Fe Precursor

The procedure was identical to that of the standard catalytic protocolwith the changes noted. A 2 mL Et₂O solution of HBAr^(F) ₄.2 Et₂O (93mg, 0.092 mmol) was added directly into a Schlenk tube equipped with astir bar and cooled to −78° C. Addition of KC₈ and subsequent work-upwas identical to the standard catalytic protocol.

TABLE 10 Attempted N₂ reduction catalysis absorption data in the absenceof an Fe precursor. Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ A0.015 0.18 1.1 B 0.005 0.06 0.4 C 0.006 0.07 0.4 D 0.008 0.09 0.6Runs with [Lutidinium][BAr^(F) ₄] as Acid

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The acid used was [Lutidinium][BAr^(F) ₄] (0.090g, 0.092 mmol).

TABLE 11 Attempted N₂ reduction catalysis absorption data using[Lutidinium][BAr^(F) ₄] as the acid. Run Absorbance Equiv NH₃/Fe % Yieldbased on H⁺ A 0.026 0.31 1.9 B 0.004 0.05 0.3 C* 0.013 0.08 0.5 D* 0.0180.11 0.7Runs with HCl as Acid

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The acid used was anhydrous HCl (46 μL of a 2.0M solution in Et₂O, 0.092 mmol) which was added without furtherdilution. The red suspension turned light yellow upon addition of acid,and subsequently precipitated a fine yellow solid. All subsequentmanipulations were carried out with this suspension.

TABLE 12 Attempted N₂ reduction catalysis absorption data usinganhydrous HCI. Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ A 0.0070.08 0.5 B 0.005 0.06 0.4 C 0.010 0.12 0.8 D 0.004 0.05 0.3Runs with Trifluoromethanesulfonic Acid as Acid

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The acid used was trifluoromethanesulfonic acid(131 μL of a 0.7 M solution in Et₂O, 0.092 mmol) which was added withoutfurther dilution. The red suspension turned light yellow-green andhomogenized upon addition of acid.

TABLE 13 Attempted N₂ reduction catalysis absorption data usingtrifluoromethanesulfonic acid. Run Absorbance Equiv NH₃/Fe % Yield basedon H⁺ A* 0.101 0.60 3.7 B* 0.069 0.41 2.6 C* 0.081 0.48 3.0 D* 0.0670.40 2.5Runs with K as Reductant

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The reductant used was K metal (4 mg, 0.1 mmol)which was added as a solid. The reaction mixture was allowed to stir at−78° C. for 40 minutes and was then warmed slowly to RT overnight. Afterthis time, a pale red-orange solution was present. Longer reaction timeswere employed to ameliorate the effect of the small surface area of theK metal.

TABLE 14 Attempted N₂ reduction catalysis absorption data using K as thereductant. Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ A 0.060 0.714.4 B 0.049 0.58 3.6 C 0.025 0.30 1.9 D 0.019 0.23 1.4Runs with CoCp*₂ as Reductant

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The reductant used was decamethylcobaltocene,CoCp*₂, (19 mg, 0.058 mmol) which was added as a solid. A heterogeneousmixture resulted at −78° C. that homogenized at room temperature,producing a yellow solution.

TABLE 15 Attempted N₂ reduction catalysis absorption data using CoCp*₂.Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ A 0.103 1.23 7.7 B 0.0620.74 4.6 C 0.045 0.27 1.7 D 0.069 0.41 2.6Runs with CrCp*₂ as Reductant

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The reductant used was decamethylchromocene,CrCp*₂, (20 mg, 0.062 mmol) which was added as a solid. The resultingsuspension darkened before gradually returning to a yellow color.

TABLE 16 Attempted N₂ reduction catalysis absorption data using CrCp*₂.Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ A 0.012 0.14 0.9 B 0.0160.19 1.2 C* 0.022 0.13 0.8 D* 0.007 0.04 0.2Supplemental Discussion

In addition to the standard precatalyst [(TPB)Fe(N₂)][Na(12-crown-4)₂]and the cationic complex [(TPB)Fe][BAr^(F) ₄], we examined the relatedTPB-containing complexes [(TPB)Fe(NH₃)][BAr^(F) ₄]¹⁵ and[(TPB)Fe(N₂H₄)][BAr^(F) ₄]¹⁵ as precatalysts for NH₃ production usingthe standard catalytic conditions. The modest attenuation in NH₃ yields(see below) relative to the yields when [(TPB)Fe(N₂)][Na(12-crown-4)₂]is used as the precatalyst may reflect less than quantitative cycling ofthe cationic derivatives to the Fe-bound N₂ species.

Runs with [(TPB)Fe(NH₃)][BAr^(F) ₄] as Precursor

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The precursor used was [(TPB)Fe(NH₃)][BAr^(F) ₄](2.9 mg, 0.002 mmol) which is an orange solid. Note that the solutionbecame homogeneous with no significant color change upon addition ofacid.

TABLE 17 Attempted N₂ reduction catalysis absorption data using[(TPB)Fe(NH₃)][BAr^(F) ₄]. Run Absorbance Equiv NH₃/Fe % Yield based onH⁺ A 0.475 5.65 35.3 B 0.487 5.80 36.2 C 0.493 5.87 36.7 D 0.472 5.6235.1Runs with [(TPB)Fe(N₂H₄)][BAr^(F) ₄] as Precursor

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The precursor used was [(TPB)Fe(N₂H₄)][BAr^(F)₄] (2.9 mg, 0.002 mmol) which is an orange solid. Note that the solutionbecame homogeneous with no significant color change upon addition ofacid.

TABLE 18 Attempted N₂ reduction catalysis absorption data using[(TPB)Fe(N₂H₄)][BAr^(F) ₄]. Run Absorbance Equiv NH₃/Fe % Yield based onH⁺ A 0.531 6.32 39.5 B 0.417 4.96 31.0 C 0.580 6.90 43.1 D 0.441 5.2532.8Runs with Fe(depe)₂N₂ as Precursor

The procedure was identical to that of the standard catalytic protocolwith the changes noted. The precursor used was Fe(depe)₂N₂ (1 mg, 0.002mmol), which is a dark red solid. Note that the solution becamehomogeneous with no significant color change upon addition of acid.

TABLE 19 Attempted N₂ reduction catalysis absorption data usingFe(depe)₂N₂. Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ A 0.0280.33 2.1 B 0.057 0.67 4.2 C 0.033 0.39 2.4 D 0.021 0.25 1.6Runs at Room Temperature

The procedure was identical to that of the standard catalytic protocolwith the changes noted. All manipulations were performed analogously tothe standard conditions at room temperature.

TABLE 20 Attempted N₂ reduction catalysis at room temperature absorptiondata. Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ A 0.158 1.88 11.7B 0.130 1.55 9.7 C 0.114 1.36 8.5 D 0.045 0.54 3.4Supplemental Discussion

Hydrazine (N₂H₄) is a possible product of the N₂ reduction catalysisdescribed but is not detected under the standard catalytic protocolusing [(TPB)Fe(N₂)][Na(12-crown-4)₂] as the precatalyst. As theexperiment below establishes, even if N₂H₄ is produced as anintermediate en route to NH₃ formation, it would be likely be consumedand converted to NH₃ under the standard catalysis conditions employed.

Inclusion of Hydrazine in a Catalytic Run with[(TPB)Fe(N₂)][Na(12-crown-4)₂]

The procedure was identical to that of the standard catalytic protocolwith the following changes noted. Hydrazine (0.12 μL, 0.004 mmol) wasadded directly to the suspension of [(TPB)Fe(N₂)][Na(12-crown-4)₂] priorto subjecting the mixture to the standard catalytic protocol. No colorchange was observed upon addition of N₂H₄. After workup the aqueoussolution was analyzed for NH₃ as described above, and also for N₂H₄ viaa literature protocol.³⁷ The results obtained show most all of the N₂H₄had been consumed, indicating that if it is produced in some amountunder the standard catalytic protocol it is unlikely to be detectable.As a control experiment it was shown that N₂H₄ was not degraded to NH₃under the standard catalytic conditions in the absence of[(TPB)Fe(N₂)][Na(12-crown-4)₂].

TABLE 21 Absorption data for a standard catalytic run in which N₂H₄ wasadded prior to [(TPB)Fe(N₂)][NA(12-crown-4)₂] acid and reductant. RunAbs. for N₂H₄ Equiv N₂H₄/Fe Abs. for NH₃ Equiv NH₃/Fe A 0.085 0.16 0.7719.18 B 0.116 0.22 0.424 5.05IR Spectral Analysis of Addition of 2 Equiv HBAr^(F) ₄.2 Et₂O to[(TPB)Fe(N₂)][Na(12-crown-4)₂], Followed by 3 Equiv KC₈

A 20 mL scintillation vial was charged with a stir bar and[(TPB)Fe(N₂)][Na(12-crown-4)₂] (8 mg, 0.0074 mmol). In a separate vial,HBAr^(F) ₄.2 Et₂O (15 mg, 0.015 mmol) was dissolved in Et₂O (1 mL).Finally, a third vial was prepared containing a suspension of potassiumgraphite (3 mg, 0.023 mmol) in Et₂O (1 mL). All three vials were chilledin the cold well to −70+/−5° C. for 30 minutes. The solution of HBAr^(F)₄.2 Et₂O was quickly added to the stirring suspension of[(TPB)Fe(N₂)][Na(12-crown-4)₂] with a glass pipette pre-cooled to −70°C. Any residue of the acid was washed with pre-chilled Et₂O (0.5 mL) andtransferred to the stirring solution. The resulting solution turnedhomogeneous. After stirring for 5 minutes, the suspension of KC₈ wasadded rapidly to the stirring solution. Any additional KC₈ was washedwith pre-chilled Et₂O (0.5 mL) and the resulting suspension wastransferred to the stirring mixture. After addition of KC₈ the solutionadopted a red color. This mixture was capped and stirred at −70° C. for40 minutes and then brought to room temperature and stirred for 10minutes. The red color persisted upon thawing to room temperature.Graphite was removed by filtration through glass filter paper. To thered solution was added 12-crown-4 (13.1 mg, 74.3 μmol) in Et₂O (1 mL)and the resulting solution was stirred for 10 minutes. The solution wasthen cooled to −70° C. for 30 minutes and stirred vigorously, leading toa red precipitate. The precipitate was collected on a filter pad and thelight orange filtrate was concentrated to dryness in vacuo. IR analysisof the precipitate (KBr pellet) showed an intense band at v_(NN)=1904cm⁻¹, identical to that of authentic [(TPB)Fe(N₂)][Na(12-crown-4)₂](v_(NN)=1905 cm⁻¹, KBr pellet). No assignable v_(NN) IR bands wereobserved for the filtrate in the window of 1700-2300 cm⁻¹. See FIG. 10.

IR and ³¹P NMR Spectral Analysis of Addition of 10 Equiv HBAr^(F) ₄.2Et₂O to [(TPB)Fe(N₂)][Na(12-crown-4)₂], Followed by 12 Equiv KC₈

A 20 mL scintillation vial was charged with a stir bar and[(TPB)Fe(N₂)][Na(12-crown-4)₂] (10.4 mg, 10.2 μmol). In a separate vial,HBAr^(F) ₄.2 Et₂O (103 mg, 102 μmol) was dissolved in Et₂O (1 mL).Finally, a third vial was prepared containing a suspension of potassiumgraphite (16.5 mg, 122 μmol) in Et₂O (1 mL). All three vials werechilled in the cold well to −70+/−5° C. for 30 minutes. The solution ofHBAr^(F) ₄.2 Et₂O was quickly added to the stirring suspension of[(TPB)Fe(N₂)][Na(12-crown-4)₂] with a glass pipette pre-cooled to −70°C. Any residue of the acid was washed with pre-chilled Et₂O (0.5 mL) andtransferred to the stirring solution. The resulting solution turnedhomogeneous. After stirring for 5 minutes, the suspension of KC₈ wasadded rapidly to the stirring solution. Any additional KC₈ was washedwith pre-chilled Et₂O (0.5 mL) and the resulting suspension wastransferred to the stirring mixture. The large amount of graphitepresent in the vial prevented the color of the resulting solution to beaccurately discerned. This mixture was capped and stirred at −70° C. for40 minutes and then brought to RT and stirred for 10 minutes. Graphitewas removed by filtration through glass filter paper. To the resultingorange solution was added 12-crown-4 (60 mg, 340 μmol) in Et₂O (1 mL)and a ³¹P NMR integration standard of triphenylphosphine (11.9 mg, 45.4μmol) in toluene (1 mL) followed by stirring for 10 minutes. Thesolution was then cooled to −70° C. for 30 minutes and stirredvigorously. No precipitate formed and volatiles were removed in vacuo.The orange powder was dissolved in THF and integration of ³¹P NMRresonances suggest the formation of (TPB)(μ-H)Fe(N₂)(H) (3.4 μmol) in30% yield. Solid-state IR analysis of the orange solid (KBr pellet)showed a strong, sharp band at v_(NN)=2073 cm⁻¹ (s), identical to thatof authentic (TPB)(μ-H)Fe(N₂)(H). Additional broad, weak bands wereobserved at 1942, 1875, 1802, 1734 cm⁻¹ that could not be assigned.³¹P{¹H} NMR (400 MHz, THF): 72.6, 63.1 ppm. See FIGS. 10 and 11.

Reactivity of (TPB)(μ-H)Fe(N₂)(H) with KC₈

A 20 mL scintillation vial was charged with a stir bar and(TPB)(μ-H)Fe(N₂)(H) (11 mg, 0.016 mmol) suspended in Et₂O (2 mL). Aseparate vial was charged with KC₈ (2.6 mg, 0.019 mmol) suspended inEt₂O (2 mL). Both vials were cooled to −70+/−5° C. and the Fe-containingvial was stirred vigorously. The suspension of KC₈ was quicklytransferred to the vial containing (TPB)(μ-H)Fe(N₂)(H) and stirred for10 minutes at low temperature. The vial was then brought to roomtemperature and the brown color of KC₈ slowly turned to black over 1hour. Graphite was filtered through a glass filter pad and the orangefiltrate was transferred to a vial containing 12-crown-4 (21.0 mg,119.17 μmol, 7.25 equivalents) and stirred vigorously at −70° C. for 10minutes. No precipitate formed and the resulting orange solution wasbrought to room temperature and concentrated to dryness in vacuo. IRanalysis of the residue (KBr pellet) showed a strong stretch atv_(NN)=2073 cm⁻¹, consistent with authentic (TPB)(μ-H)Fe(N₂)(H) (2073cm⁻¹, KBr pellet). ¹H NMR analysis was consistent with the presence ofpredominately (TPB)(μ-H)Fe(N₂)(H) and minor amounts of unidentifiedparamagnetic species.

Reactivity of (TPB)(μ-H)Fe(N₂)(H) with HBAr^(F) ₄.2 Et₂O

A 20 mL scintillation vial was charged with a stir bar and(TPB)(μ-H)Fe(N₂)(H) (9 mg, 0.014 mmol) suspended in Et₂O (2 mL). Aseparate vial was charged with HBAr^(F) ₄.2 Et₂O (15 mg, 0.015 mmol,1.08) suspended in Et₂O (2 mL). Both vials were cooled to −70+/−5° C.and the Fe-containing vial was stirred vigorously. The solution ofHBAr^(F) ₄. 2 Et₂O was quickly transferred to the vial containing(TPB)(μ-H)Fe(N₂)(H) and stirred for 10 minutes at low temperature. Thevial was then brought to room temperature and no noticeable color changewas observed over 1 hour. The solution was concentrated to dryness invacuo and the remaining residue was analyzed with IR spectroscopy (KBrpellet) which showed a strong stretch at v_(NN)=2073 cm⁻¹, consistentwith authentic (TPB)(μ-H)Fe(N₂)(H) (2073 cm⁻¹, KBr pellet). The residuewas then re-dissolved in C₆D₆ and analyzed by ¹H NMR spectroscopy whichshowed (TPB)(μ-H)Fe(N₂)(H) with minor amounts of unidentifiedparamagnetic species and resonances from the BAr^(F) ₄ anion. Completeconsumption of (TPB)(μ-H)Fe(N₂)(H) to unidentified paramagnetic specieswas observed after 12 hours at room temperature.

Identification of [(TPB)Fe(NH₃)][BAr^(F) ₄] from Protonation of[(TPB)Fe(N₂)][Na(12-crown-4)₂]

[(TPB)Fe(N₂)][Na(12-crown-4)₂] (5 mg, 0.005 mmol) was dissolved in 2 mLof THF and cooled to −78° C. This dark red solution was added dropwiseto a similarly cooled 2 mL THF solution of HBAr^(F) ₄.2 Et₂O (29 mg,0.029 mmol) with stirring. The resulting yellow-orange solution wasallowed to stir for 10 minutes at low temperature before being warmed toroom temperature and stirred for an additional 40 minutes.1,8-Bis(dimethylamino)naphthalene (6 mg, 0.029 mmol) was added and thesolution was allowed to stir for 15 minutes with no noticeable colorchange. Volatiles were removed from the solution and the resultingyellow residue was taken up in THF-d₈. The presence of[(TPB)Fe(NH₃)][BAr^(F) ₄]¹⁵ was determined by comparison of the ¹H NMRspectrum with that of an authentic sample prepared as recently reported.Additionally, a capillary insert of the (TPB)FeMe¹⁵ in THF-d₈ was addedto the NMR sample which allowed for crude measurements of the yield of[(TPB)Fe(NH₃)][BAr^(F) ₄], a species tentatively assigned as[(TPB)Fe][BAr^(F) ₄]¹⁵ and the total amount of S=3/2 TPB species asroughly 30%, 50%, and 100% respectively. Note that there is likely asignificant degree of error on these measurements due to the broadparamagnetic peaks used for integration. See FIG. 5.

Identification of H₂ in Standard Catalytic Runs

The catalytic runs were performed according to the standard procedure.Prior to the vacuum transfer of volatiles, the solutions inside of theSchlenk tubes were frozen. The ground glass joint of the Schlenk tubewas then sealed with a rubber septum and the head space between theTeflon stopcock of the Schlenk tube and the septum was evacuated. Thishead space was left under static vacuum and the Teflon stopcock of thereaction vessel was opened after which a 10 mL aliquot of the headspacewas sampled through the septa via a gas-tight syringe. This sample wasthen analyzed for hydrogen with an Agilent 7890A gas chromatograph usinga thermal conductivity detector. After H₂ analysis, the reaction vesselwas sealed and subjected to the standard analysis for NH₃. As some H₂leakage is unavoidable by the procedure used, these values representlower limits of the H₂ yield.

TABLE 22 Absorption and gas chromatograph integration data for standardcatalytic runs. Run Absorbance Equiv NH₃/Fe % Yield based on H⁺ % Yieldof H₂ A 0.500 5.95 37.2 30 B 0.365 4.34 27.1 40Identification of H₂ in Runs without an Fe Precursor

A Schlenk tube was charged with a stir bar and a suspension of KC₈ (14mg, 0.100 mmol) in Et₂O (0.5 mL). The Schlenk tube was then fitted witha Teflon stopcock, but not sealed. The ground glass joint on the Schlenktube was sealed with a rubber septum. This reaction vessel was thencooled to −78° C. A pre-cooled solution of HBAr^(F) ₄.2 Et₂O (92 mg,0.092 mmol) in Et₂O (2 mL) was then syringed directly into the reactionvessel with stirring after which the vessel was rapidly sealed with itsTeflon stopcock. The reaction was allowed to stir for 40 minutes at lowtemperature before the headspace between the Teflon valve and the septawas evacuated. After evacuation, the Teflon stopcock was opened and a 10mL aliquot of the headspace was sampled via a gas tight syringe. Thissample was then analyzed for hydrogen with an Agilent 7890A gaschromatograph using a thermal conductivity detector. The yield ofhydrogen observed, based on proton-equivalents was 66% and 88% for eachof two runs, respectively. As some H₂ leakage is unavoidable by theprocedure used, these values represent lower limits of the H₂ yield.

Referring to FIG. 6, the middle spectrum was generated by dissolving[(TPB)Fe(N₂)][Na(12-crown-4)₂] (4 mg, 0.004 mmol) in 250 μL of 2-MeTHFto generate a deep red solution. This solution was then transferred toan EPR tube and frozen within a liquid N₂ cooled cold well. Another 250μL 2-MeTHF solution of HBAr^(F) ₄.2 Et₂O (38 mg, 0.037 mmol) wasprepared and carefully layered onto the frozen solution of[(TPB)Fe(N₂)][Na(12-crown-4)₂] in the EPR tube. The layered solutionswere then frozen. At this time, the solutions were warmed until barelythawing (−140° C.) and mechanically mixed with a long needle. Uponmixing, the red color of [(TPB)Fe(N₂)][Na(12-crown-4)₂] disappeared anda yellow solution was obtained. The solution was frozen and the EPRspectrum shown was obtained at 77 K. Immediately prior to obtaining thisspectrum, a spectrum of a sample of [(TPB)Fe(N₂)][Na(12-crown-4)₂] (4mg, 0.004 mmol) in 500 μL of 2-MeTHF was obtained under identicalconditions. Both spectra were then doubly integrated and compared toobtain an integrated yield for the formation of the new S=½ speciesshown in the figure. Repetition of this experiment in triplicateprovided an average yield of ˜90%.

Referring to FIG. 6, the bottom spectrum was generated by dissolving(TPB)Fe≡N(p-C₆H₄OMe)¹⁴ (3 mg, 0.004 mmol) in 250 μL of 2-MeTHF. Thissolution was then cooled to −78° C. and then mixed with a similarlycooled 250 μL solution of [Fc][BAr^(F) ₄] (Fc=ferrocenium) (4 mg, 0.004mmol) with rapid stirring. Upon mixing the dark blue color of[Fc][BAr^(F) ₄] disappeared and the dark green color of(TPB)Fe≡N(p-C₆H₄OMe) lightened slightly to green-yellow. The solutionwas allowed to stir for an additional 10 minutes then transferred to aquartz EPR tube and frozen. The EPR spectrum shown was obtained at 77 K.Note that the asterisk denotes a signal of an unknown S=½ component ofthe product mixture.

EXAMPLE 2 Catalytic Reduction of N₂ to NH₃ b an Fe—N₂ Complex Featuringa C-Atom Anchor

Abstract

While recent spectroscopic studies have established the presence of aninterstitial carbon atom at the center of the iron-molybdenum cofactor(FeMoco) of MoFe-nitrogenase, its role is unknown. In this Example wepursue Fe—N₂ model chemistry to characterize whereby this C-atom(previously denoted as a light X-atom) may provide a flexible transinteraction with an Fe center to expose an Fe—N₂ binding site. ThisExample, describes Fe complexes of a new tris(phosphino)alkyl (CP^(iPr)₃) ligand featuring an axial carbon donor. It is established that theiron center in this scaffold binds dinitrogen trans to theC_(alkyl)-atom anchor in three distinct and structurally characterizedoxidation states. Fe—C_(alkyl) lengthening is observed upon reduction,reflective of significant ionic character in the Fe—C_(alkyl)interaction. The anionic (CP^(iPr) ₃)FeN₂ ⁻ species can befunctionalized by a silyl electrophile to generate (CP^(iPr)₃)Fe—N₂SiR₃. (CP^(iPr) ₃)FeN₂ ⁻ also functions as a modest catalyst forthe reduction of N₂ to NH₃ when supplied with electrons and protons at−78° C. under 1 atm N₂ (4.6 equiv NH₃/Fe).

Introduction

The biological reduction of atmospheric N₂ to NH₃ is a fascinating yetpoorly understood transformation that is essential to life.¹ Theiron-molybdenum cofactor (FeMoco) of MoFe nitrogenase catalyzes N₂reduction and has been extensively studied.² This cofactor has attractedthe attention of inorganic and organometallic chemists for decades whohave sought inspiration to explore the ability of synthetic iron andmolybdenum complexes to bind and reduce dinitrogen.^(3,4,5,6) Advancesin the past decade have included two molybdenum systems that facilitatecatalytic turnover of N₂ to NH₃ in the presence of inorganic acid andreductant sources,^(7,8,9) and iron complexes that support a range ofN_(x)H_(y) ligands relevant to nitrogen fixation,^(10,11,12,13) effectreductive N₂ cleavage,^(14,15) and facilitate N₂functionalization.^(16,17,18)

The presence of an interstitial light atom in the MoFe nitrogenasecofactor was established in 2002,¹⁹ and structural, spectroscopic, andbiochemical data have more recently established its identity as aC-atom.²⁰ The role of the C-atom is unknown. This state of affairsoffers an opportunity for organometallic chemists to undertake modelstudies that can illuminate plausible roles for this interstitialC-atom, and hence important aspects of the mechanism of N₂ reductioncatalysis. In particular, Fe-alkyl complexes that are more ionic innature than a prototypical transition metal-alkyl may be relevant tomodeling the Fe—C_(interstitial) interaction of the possible N₂ bindingsite in the cofactor (FIG. 12).

These results support a possible role played by the interstitial C-atomis to provide a flexible Fe—C_(interstitial) interaction that exposes anFe—N₂ binding site on a belt iron atom trans to the Fe—C linkage (FIG.12).^(3,15,21,22,23) Subsequent modulation of the Fe—C interaction andhence the local Fe geometry as a function of the N₂ reduction statemight enable the Fe center to stabilize the various N_(x)H_(y)intermediates sampled along a pathway to NH₃.

To characterize Fe-mediated N₂ reduction, our group has employedphosphine-supported Fe complexes in approximately trigonal geometries(pseudotetrahedral, trigonal pyramidal, or trigonal bipyramidal) to bindand functionalize dinitrogen. Tripodal trisphosphine ligands featuringan axial donor (X=N, Si, B) and aryl backbones have been used to canvassthe ability of low-valent iron in such geometries to bind and activatedinitrogen (FIG. 13).^(23,24,25) The (TP^(iPr)B)Fe-system(TP^(R)B=tris(o-phosphinoaryl)borane) has proven rich in this context,and has been shown in Example 1 to be an effective catalyst for thereduction of N₂ to NH₃ in the presence of proton and electron sources atlow temperature and 1 atm N₂.²¹ An important feature of the(TP^(iPr)B)Fe-system is the presence of a flexible Fe—Binteraction.^(15,25) This flexibility may facilitate the formation ofintermediates featuring Fe—N_(X) π-bonding (e.g., Fe═NNH₂, Fe≡N, Fe═NH)during catalysis. Whether the aforementioned hypothesis concerning ahemi-labile role for the interstitial C-atom of FeMoco is correct ornot, these inorganic model studies demonstrate the principles ofcoordination chemistry are concerned.

To extend our studies to systems that place a C-atom in a position transto an Fe—N₂ binding site we have sought related ligand scaffolds thatfeature a C-atom anchor. In designing these scaffolds we havehypothesized that the proposed flexibility of the Fe—C linkage in theFeMo cofactor may be facilitated by the ability of the environmentaround the interstitial carbide—five additional electropositive Featoms—to stabilize developing negative charge on the carbon. With thisin mind we reported iron complexes of a tris(phosphino)alkyl ligandwhose axial carbon binding site is flanked by three electropositivesilyl groups (FIG. 13) which may play a role in stabilizing thesubstantial ionic character of this Fe—C_(alkyl) bond (FIG. 12).²²

In this Example, we report a new tris(phosphino)alkyl ligand, (CP^(iPr)₃), featuring aryl linkers bound to the axial carbon. We reasoned thatpossible delocalization of negative or positive charge buildup into thearyl π-system would allow for increased flexibility in the Fe—C bond;this flexibility is expected to facilitate possible catalytic N₂functionalization and reduction, as discussed above. Additionally, asthis ligand is closely structurally related to the SiP₃, TPB, and NP₃ligands whose iron coordination chemistry we have extensively explored,Fe complexes of CP₃ ^(iPr) are of obvious comparative interest and wouldbe particularly beneficial with regard to considering the role anFe—C_(interstitial) interaction might play in facilitating N₂ bindingand reduction within the cofactor. To this end, we embarked on thesynthesis of the new ligand (CP^(iPr) ₃)H and the development of itsFe—N₂ chemistry.

Results and Discussion

Ligand Synthesis. Whereas the ligands (SiP^(iPr) ₃)H and TP^(iPr)B arestraightforward to synthesize by the addition of lithiatedo-phosphinophenyl precursors to HSiCl₃ and BCl₃,^(24,26) the preparationof (CP^(iPr) ₃)H via an analogous method by addition ofphosphinoaryllithium moieties to a C₁ source (e.g., triple addition todimethylcarbonate followed by deoxygenation of the resultanttriarylmethanol product) has proven ineffective in our hands. However,an orthogonal synthetic approach based on elaboration of an initiallyformed triarylmethane scaffold afforded a viable approach to thepreparation of (CP^(iPr) ₃)H on a multigram scale and in reasonableyields. This synthesis of (CP^(iPr) ₃)H follows an approach inspired bya previously reported synthesis of Ph₂P(o-C₆H₄CH₂C₆H₄-o)PPh₂,²⁷ andhinges on the sequential formation and cleavage of two diaryliodoniumions to give the tris(2-halophenyl)methane precursor (5) (Scheme 1).

The synthesis of o-iodotriphenylmethane has been reported²⁸ and isreadily effected in three steps from commercially available2-nitrobenzaldehyde on a 20-gram scale. Cyclization of this species tothe diaryliodonium bromide salt (2) is accomplished by a reportedtechnique.²⁹ Slow but clean ring-opening of 2 by CuBr and [TBA][Br] inacetonitrile gives 2-bromo-2′-iodotriphenylmethane (3). The2-bromo-2′-iodotriphenylmethane species was targeted rather than2,2′-diiodotriphenylmethane in order to mitigate the possibility ofcomplications from excessive oxidation in the next step.

Formation of a second diaryliodonium cation as its iodide salt followsvia an analogous procedure to regioselectively generate 4, which can bestraightforwardly decomposed to 2-bromo-2′,2″-diiodotriphenylmethane (5)by heating to 200° C. for 15 minutes under an inert atmosphere. Eachstep in the synthesis of 5 from o-iodotriphenylmethane can beaccomplished in 75% yield or more (overall yield: 38% over five steps).

Lithiation of 5 with six equiv of tert-butyllithium at −78° C. followedby treatment with three equiv of diisopropylphosphine chloride gives thedesired tris(o-diisopropylphosphinophenyl)methane, (CP^(iPr) ₃)H (1) in67% yield (Scheme 1). The protonated form of the ligand, 1, ischaracterized by a single peak in its phosphorus NMR spectrum at −9.1ppm. The ¹H NMR spectrum, while indicative of three-fold symmetry, alsoshows features suggestive of a rigid ligand scaffold where rotationabout the phosphine-carbon bonds is hindered; in particular, fourmagnetically inequivalent sets of resonances are observed for theisopropyl methyl hydrogens. Additionally, the central C—H methine protonis shifted markedly downfield (8.15 ppm) and manifests as a quartet dueto through-space coupling to the three phosphorus atoms. Similar NMRproperties were observed for the central methine proton in a relatedtrisphosphine ligand based on a tris(indolyl)methane scaffold.³⁰

Metallation at Iron and Precursor Complexes. We initially sought toeffect metallation of 1 by first deprotonating it to give an alkalimetal complex followed by transmetallation with an iron (II) halide orother transition metal precursor. To our frustration, 1 provedunexpectedly difficult to deprotonate even with very strong bases suchas benzyl potassium and Schlosser's base,³¹ perhaps due in part to thesteric protection of the methine proton; additionally, the acidity ofthis proton is likely not as high as for bare triphenylmethane since theligand bulk limits the extent to which the aryl rings can approach acoplanar configuration to afford resonance stabilization of a resultingcarbanion.³² Furthermore, the strategy used for metallation of the(SiP^(iPr) ₃)H ligand on iron—using methyl Grignard with FeCl₂ togenerate a methyl iron complex which then eliminates methane withconcomitant formation of the iron-silicon bond²⁴—was not effective for(CP^(iPr) ₃)H. It appeared to instead result in reduction of ironwithout the formation of the desired iron-carbon bond. Thus, it wasnecessary to develop a different protocol for the formation of a(CP^(iPr) ₃)Fe-complex featuring an iron-carbon bond.

Combining 1 and iron(II) iodide in toluene cleanly affords thetetracoordinate, K2-bisphosphine diiodide high-spin iron(II) complex (6)as a yellow powder (Scheme 2). Its solid-state structure (FIG. 14) showsa tetrahedral environment at the iron center and a bidentate bindingmode for the ligand. One-electron reduction of 6 in benzene or tolueneusing a range of reagents including sodium amalgam, potassium graphite,or alkylmagnesium/lithium reagents, results in the formation of the deepbrick-red four-coordinate iron(I) complex {(CP^(iPr) ₃)H}FeI (7). Thebromide congener, {(CP^(iPr) ₃)H}FeBr (8), is analogously prepared andhas been crystallographically characterized (FIG. 14); its most notablefeature is the endo orientation of the unactivated methine C—H. Thisproton is located within the ligand cage pointed nearly linearly towardsthe iron center. Both 7 and 8 are unstable with respect todisproportionation to Fe(0), (CP^(iPr) ₃)H, and {(CP^(iPr) ₃)H}FeX₂(X=I, Br), especially in coordinating solvents. However, if appropriateconditions are employed, 7 is sufficiently long-lived to be generatedand used without further purification for subsequent reactions.

Further reduction of 7 with sodium metal in a 5:1 mixture of Et₂O andDME at −78° C. causes formal insertion of the Fe center into the C—Hbond of the (CP^(iPr) ₃)H ligand and uptake of atmospheric N₂ to giveyellow, diamagnetic (CP^(iPr) ₃)Fe(H)(N₂) (9). The position of the ironhydride is identifiable in the XRD difference map of 9, as is thepresence of an Fe—C bond at 2.155(2) Å (FIG. 14). IR data for 9 show astrong N—N vibration at 2046 cm⁻¹ and an Fe—H vibration at 1920 cm⁻¹;The properties of 9 can be compared to the isostructural (SiP^(iPr)₃)Fe(H)(N₂) and [(NP^(iPr) ₃)Fe(H)(N₂)]⁺ complexes^(23,33) and otherclosely related species such as {[P(CH₂CH₂P^(i)Pr₂)₃]Fe(H)(N₂)}⁺;³⁴ thevibrational and metrical properties of the N₂ ligand suggest a moreactivated dinitrogen moiety in 9 relative to its congeners.

Deprotonation of 9 to afford (CP^(iPr) ₃)FeN₂ ⁻ was canvassed but provedunsuccessful. A more circuitous but ultimately effective route to(CP^(iPr) ₃)FeN₂ ⁻ proceeded via treatment of 9 with anhydrous HCl inEt₂O to afford dark red-orange (CP^(iPr) ₃)FeCl (10) in good yield(Scheme 2). The crystal structure of 10 was not reliably determined dueto its propensity to crystallize in a cubic space group with extensivewhole molecule disorder. Complex 10 is paramagnetic and its roomtemperature solution magnetic moment of 4.9 μ_(B) is suggestive of ahigh-spin, S=2 ground state. A lower spin state might have beenreasonably anticipated to arise from a presumably strong-field ligandset comprised of three diisopropylarylphosphines and an alkyl group. Forcomparison, (SiP^(iPr))₃FeCl exhibits an intermediate S=1 groundstate.²⁴ The C_(alkyl) anchor in 10 thereby appears to be a weaker-fielddonor than the silyl anchor in (SiP^(iPr))₃FeCl.

Synthesis and characterization of the {(CP^(iPr) ₃)FeN₂}^(n) (n=0, −1,+1) series. Reduction of the chloride precursor 10 affords entry intothe desired series of trigonal bipyramidal iron dinitrogen complexes.Stirring 10 over sodium metal in THF produces the neutral low-spin Fe(I)complex (CP^(iPr) ₃)FeN₂ (11) (υ(NN)=1992 cm⁻¹) (Scheme 3). Complex 11is low-spin and paramagnetic (S=½); it has been crystallographicallycharacterized (FIG. 15) and shows a distortion from trigonal symmetrywith one widened P—Fe—P angle (132.5°), as expected due to theJahn-Teller active ground state. The N₂ vibrational frequency and N—Nbond length (1.134(4) Å) show that the dinitrogen ligand in this complexis somewhat more activated than that in the isoelectronic (SiP^(iPr)₃)FeN₂ complex (υ(NN)=2003 cm⁻¹, N—N=1.1245(2) Å) or in the neutralFe(0) complex (TP^(iPr)B)FeN₂ (υ(NN)=2011 cm⁻¹).^(17,25) Thesedifferences are relatively small and as such are difficult to reliablyinterpret. But given the fact that (CP^(iPr) ₃) appears to have aweaker-field donor set than (SiP^(iPr) ₃) according to the observedground spin states of (CP^(iPr) ₃)FeCl (S=2) and (SiP^(iPr) ₃)FeCl(S=1), one might have reasonably anticipated (SiP^(iPr) ₃)FeN₂ to have alower υ(NN) than (CP^(iPr) ₃)FeN₂.

Both a one-electron oxidation and a one-electron reduction of 11 areaccessible (FIG. 16). The Fe(II/I) couple appears at −1.20 V (vs Fc/Fc⁺)and is quasi-reversible; the current in the cathodic wave is diminishedand an irreversible reduction wave appears at −1.65 V. This is verysimilar electrochemical behavior to what has been documented for(SiP^(iPr) ₃)FeN₂ and suggests that the same phenomenon is responsiblefor the observations in this system¹⁷—that is, N₂ coordinates reversiblyto the {(CP^(iPr) ₃)Fe}⁺ complex; partial loss of N₂ upon oxidation of(CP^(iPr) ₃)FeN₂ is likely responsible for the quasi-reversibility ofthe (II/I) couple, and the reduction at −1.65 V is most reasonablyattributed to the cationic species {(CP^(iPr) ₃)Fe(L)}⁺ (where L may beTHF, or may be a vacant site), which then takes up N₂ upon reduction.The Fe(I/0) couple is fully reversible, consistent with the formation ofa stable (CP^(iPr) ₃)FeN₂ ⁻ anion. This reduction occurs at an unusuallynegative potential (−2.55 V vs Fc/Fc⁺). For comparison, the reduction of(SiP^(iPr) ₃)FeN₂ to (SiP^(iPr) ₃)FeN₂ ⁻ occurs at −2.2 V.¹⁷

The Fe—N₂ adduct triad {(CP^(iPr) ₃)FeN₂}^(n) (n=0 (11), −1 (12), +1(13)) proved synthetically accessible. Treatment of 10 with an excess ofpotassium graphite (KC₈) in Et₂O results in immediate reduction to thevery dark brown-blue CP^(iPr) ₃FeN₂ ⁻ anion (12). The IR spectrum of athin film deposited from diethyl ether solution shows a υ(NN) vibrationat 1870 cm⁻¹, suggestive of a close ion pair with the potassium ioncapping the N₂ moiety. Accordingly, treatment of the potassium complexwith two equivalents of 12-crown-4 results in the formation of[(CP^(iPr) ₃)FeN₂][K(12-crown-4)₂] (12[K(12-crown-4)₂]) with a shift ofthe υ(NN) vibration to 1905 cm⁻¹. The anion has beencrystallographically characterized (FIG. 15) as its K(Et₂O)₃ salt,[(CP^(iPr) ₃)FeN₂][K(Et₂O)₃] (12[K(Et₂O)₃]; the bulk material afterdrying is solvated by 0.5 molecules of Et₂O per anion,12[K(Et₂O)_(0.5)]).

Oxidation of 11 with one equivalent of [Cp*₂Fe][BAr^(F) ₄](Ar^(F)=3,5-trifluoromethylphenyl; Cp*=pentamethylcyclopentadienide) inEt₂O gives rise to [(CP^(iPr) ₃)FeN₂][BAr^(F) ₄] (13) as an orangecrystalline solid, which has also been structurally characterized (FIG.15). The dinitrogen ligand in 13 (υ(NN)=2128 cm⁻¹), is labile and insolution under an N₂ atmosphere appears to be in equilibrium with asolvated or vacant cation [(CP^(iPr) ₃)Fe(L)]⁺; in addition to theelectrochemical properties discussed above, evidence from UV-Visspectroscopy is consistent with the loss of coordinated N₂ under vacuum.

Whereas a related series was accessible for the silyl-anchored{(SiP^(iPr) ₃)FeN₂}^(n) system (n=0, +1, −1),¹⁷ only the anion(C^(Si)P^(Ph) ₃)FeN₂ ⁻ proved accessible for the C_(alkyl)-anchoredsystem.²² Hence, the present {(CP^(iPr) ₃)FeN₂}^(n) series allows for adirect comparison of how the anchoring atom (Si vs C) responds acrossthree redox states when positioned trans to an N₂ ligand of anisostructural trigonal bipyramidal framework.

In the case of the {(SiP^(iPr) ₃)FeN₂}^(n) series, the Fe—Si bonddistance decreases upon reduction from 2.298(7) Å in the (SiP^(iPr)₃)FeN₂+ cation to 2.2526(9) Å in the (SiP^(iPr) ₃)FeN₂ ⁻ anion. Indirect contrast, the Fe—C bond distance in {(CP^(iPr) ₃)FeN₂}^(n)increases upon reduction, from 2.081(3) Å in 13 to 2.152(3) Å in 11 to2.1646(17) Å in 12. The different responses manifest in these twosystems may be due to the electropositive silicon atom binding morestrongly to the more electron-rich iron, whereas the moreelectronegative C_(alkyl) binds more strongly to the higher-valent, moreelectron-deficient iron center.

Notably, the overall change in the bond length is greater in theCP^(iPr) ₃ case (0.084 Å from 13 to 12) than for the more covalentSiP^(iPr) ₃ system, where the overall change is only 0.045 Å despite thelonger total bond length. This suggests a greater degree of flexibilityin the Fe—C_(alkyl) interaction. A similar conclusion was drawn for the{(C^(Si)P^(Ph) ₃)Fe(CO)}^(n) (n=+1, 0, −1) series, where an even morepronounced Fe—C lengthening was observed upon reduction.²²

TABLE 23 Select characterization data for the Fe—N₂ adducts{(CP^(iPr))₃FeN₂}^(n) and {(S^(iPr) ₃)FeN₂}^(n) (n = −1, 0, 1). X = C,Si^(a) [X—Fe—N2]^(−b) X—Fe—N2 [X—Fe—N2]⁺ Fe—C (Å) 2.1646(17) 2.152(3)2.081(3) Fe—Si (Å) 2.2526(9) 2.2713(6) 2.298(7) Fe—N_(x=c) (Å)1.7397(16) 1.797(2) 1.864(7) Fe—N_(x=si) (Å) 1.763(3) 1.8191(1) 1.914(2)ν(N₂)_(x=c) (cm⁻¹) 1870 1992 2128 ν(N₂)_(x=si) (cm⁻¹) 1891 2003 2143spin state S = 0 S = ½ S =1 ^(a)All data tabulated for X = Si is takenfrom reference 17. ^(b)For X = C, data provided is for the [K(Et₂O)₃]⁺salt (Figure 15). For X = Si, data provided is for the [Na(THF)₃]⁺ salt.

In the case of the (TP^(iPr)B)Fe system, a highly flexible Fe—Binteraction has been observed as a function of the ligand positionedtrans to the B-atom that may be important to its success in activatingN₂ in both stoichiometric and catalytic reactions.^(15,21,35) However,an analogous series of N₂ complexes has not been characterized to allowfor direct comparison. Whereas the anion [(TP^(iPr)B)]FeN₂]⁻ has beenstudied by X-ray crystallography (Fe—B=2.311(2) Å), the [(TP^(iPr)B)Fe]⁺cation does not coordinate N₂ at atmospheric pressure, and attempts toobtain the crystal structure of neutral (TP^(iPr)B)FeN₂ have beenunsuccessful.^(25,35) Nonetheless, our chemical intuition is that theFe—B linkage in (TP^(iPr)B)Fe will be appreciably more flexible than theFe—C linkage in (CP^(iPr) ₃)Fe.

The C_(alkyl)-Fe interactions in both (CP^(iPr) ₃)FeN₂ ⁻ (12) and(C^(Si)P^(Ph) ₃)FeN₂ ⁻ reflect a higher degree of ionic character thanin a prototypical Fe—C_(alkyl) bond, with (C^(Si)P^(Ph) ₃)FeN₂ ⁻ beingmost striking in this context.²² Comparative DFT studies of(C^(Si)P^(Ph) ₃)FeN₂ ⁻ and (CP^(iPr) ₃)FeN₂ ⁻ including NBO analyses,support this view,^(22,36) predicting strong polarization of the σ-bondpair towards the C-atom (23% Fe/77% C in (S^(Si)P^(Ph) ₃)FeN₂ ⁻; 27%Fe/73% C in (CP^(iPr) ₃)FeN₂ ⁻) (FIG. 17). As expected, the Fe—C bond in12 is slightly more covalent than that in (C^(Si)P^(Ph) ₃)FeN₂ ⁻, wherethe axial carbon is flanked by electropositive silicon atoms.Comparative NBO analyses for (C^(Si)P^(Ph) ₃)FeN₂ ⁻, (SiP^(iPr) ₃)FeN₂⁻.

Second-order perturbation analysis from an NBO calculation indicates thepresence of stabilizing donor-acceptor interactions between filled andvirtual orbitals, representing deviations from a simple Lewis structuredescription due to electronic delocalization.³⁶ In the case of 12,significant interactions between the filled Fe—C_(alkyl) σ bond and π*orbitals of the aryl rings (C_(ipso)-C_(ortho)) are evident (FIG. 17).Three primary donor-acceptor interactions (one to each ring) arelocated, representing stabilizations of 6.70 kcal/mol, 5.99 kcal/mol,and 5.95 kcal/mol. This result supports that stabilization of thenegative charge on carbon by delocalization onto the aryl rings is atleast partially responsible for the observed ionic character of the Fe—Cbond, and hence for its increased flexibility. A similar stabilizationof ionic character at an N₂—Fe—C_(interstitial) site of the cofactor mayfacilitate N₂ binding.

Reactivity Studies. To compare the reactivity of (CP^(iPr) ₃)FeN₂ ⁻ atthe bound N₂ ligand with (SiP^(iPr) ₃)FeN₂ ⁻, (C^(Si)P^(Ph) ₃)FeN₂ ⁻,and (TP^(iPr)B)FeN₂ ⁻, treatment of 12 with TMSCl at −78° C. wasexamined and afforded the diamagnetic diazenido complex (CP^(iPr)₃)FeN₂SiMe₃ (14) (υ(NN)=1736 cm⁻¹). This product, though it has not beenstructurally characterized, is spectroscopically similar to thoseobtained for the structurally related Si- and B-anchoredsystems.^(15,17)

More interesting is the comparative behavior of (CP^(iPr) ₃)FeN₂ ⁻ ontreatment with proton/electron equivalents at low temperature. Numerousstudies have explored the possibility of Fe—N₂ protonation/reduction torelease ammonia,^(3,4,5,6,37) which in all but one case²¹ afforded lowchemical yields of NH₃ (ca. ≦10% per Fe in one step; 35% per Fe overallin two independent synthetic steps¹⁴). The C-anchored system(C^(Si)P^(Ph) ₃)FeN₂ ⁻ (FIG. 13) follows a similar trend, affordingnegligible NH₃ on treatment at low temperature with [H(Et₂O)₂][BAr^(F)₄] and KC₈. The Si-anchored system (SiP^(iPr) ₃)FeN₂ ⁻ also affordssub-stoichiometric NH₃ yields (35% per Fe) when similarly treated, andinstead produces some N₂H₄ (˜45% per Fe) when H(Et₂O)BF₄ and CrCl₂ areemployed.²⁴

By contrast, cooling a solution of 12[K(Et₂O)_(0.5)] in Et₂O at −78° C.followed by the addition of 40 equiv KC₈ and then 38 equiv[H(Et₂O)₂][BAr^(F) ₄] leads to the formation of 4.6±0.8 equiv NH₃ (230%per Fe; average of 8 runs; Eq. 1), a yield that establishes a modestdegree of N₂ reduction catalysis at low temperature. No N₂H₄ isobserved. With 12[K(12-crown-4)₂] as the catalyst, the NH₃ yield isslightly lower at 3.5±0.3 equiv. NH₃ quantification was carried out byUV-Vis using the indophenol protocol³⁸ as recently described in detailfor the (TP^(iPr)B)FeN₂ ⁻ catalyst system.²¹ The total NH₃ product yieldis lower for (CP^(iPr) ₃)FeN₂ ⁻ than that which was obtained for(TP^(iPr)B)FeN₂ ⁻ when acid was added prior to the reductant. Thesignificance of these modest differences is unclear, especially giventhe extreme air-sensitivity of the catalysts and the low turnovernumbers. The order of addition of reagents has a minor effect; reversingthe order and adding first acid, then reductant to 12[K(Et₂O)_(0.5)]decreases the yield to 3.8±0.6 equiv. NH₃ per Fe. In side-by-sidecomparisons using the same batches of reagents (KC₈ and[H(Et₂O)₂][BAr^(F) ₄]) and the same order of addition (reductant addedfirst), 12[K(Et2O)0.5] afforded 4.4±0.2 equiv. NH3 per Fe, as comparedto 5.0±1.1 for (TPiPrB)FeN2- and 0.8±0.4 for (SiPiPr3)FeN2-.

Treatment of 12[K(Et₂O)_(o.5)] with 10 equivalents of [H(Et₂O)₂][BAr^(F)₄] in the absence of added reductant generates negligible ammonia (<0.05equivalents), verifying that both acid and reductant are necessary forthe production of substantial amounts of NH₃.

In order to examine possible reasons for the limited turnover forammonia production with this system, we sought to determine the fate ofthe precatalyst over the course of the experiment. An analysis of theiron-containing products of a reaction mixture using 10 equivalents of[H(Et₂O)₂][BAr^(F) ₄] and 12 equivalents of KC₈ (FIG. 18) identified themajor iron-containing product as (CP^(iPr) ₃)FeN₂ (11), which is readilyreduced by KC₈ even at low temperature to reform the precatalyst 12.However, a significant amount of (CP^(iPr) ₃)Fe(N₂)(H) (9) is alsopresent; 9 is not catalytically competent, generating no detectableammonia when subjected to the catalytic conditions, and its formation islikely an important limiting factor in the catalyst performance. Anotheridentifiable species by ¹H NMR is (CP^(iPr) ₃)FeCl (10). Despite ourefforts to remove all Cl⁻ in the preparation of [H(Et₂O)₂][BAr^(F) ₄],the large excess of acid employed in this experiment likely ensures anon-negligible Cl⁻ impurity that may also attenuate catalyst activity.The ¹H NMR is generally indicative of another diamagnetichydride-bearing species.

Further product analysis using the full catalytic conditions (38equivalents of [H(Et₂O)₂][BAr^(F) ₄] and 40 equivalents of KC₈ withrespect to the catalyst), showed that increasing amounts of (CP^(iPr)₃)Fe(N₂)(H) (9) are formed as the system goes through more turnovers,corroborating the idea that this species serves as a catalyticallyinactive sink which builds up throughout the reaction. Integration ofthe NMR spectrum of such a reaction mixture against an internal standardsuggests that approximately 70% of the catalyst has been converted to 9;even at this point, however, some active catalyst remains in the form of11 (FIG. 18). The unknown hydride species present in the aforementionedreaction mixture derived from fewer equivalents of acid and reductant isno longer observed.

Notably, in neither of these experiments was any free ligand 1 (nor anyligand decomposition product) detected; it appears that all of the ironpresent remains ligated by the CP^(iPr) ₃ ligand. This lack ofdegradation is promising, and suggests that improvements to the N₂reduction catalysis, in terms of turnover number, may yet prove possibleif the formation of terminal hydride 9 can be limited by modification ofeither the ligand scaffold and/or the catalytic conditions. Indeed, itmay be that biological nitrogenases are designed to avoid catalyticallyinactive hydride sinks by being themselves modest hydrogenases.³⁹ Acluster approach would be a particularly good design in this context.⁴⁰

CONCLUSIONS

To conclude, we have synthetically introduced the tripodal (CPiPr3)Hligand and have prepared and structurally compared its {(CP^(iPr)₃)FeN₂}^(n) complexes (n=0, −1, +1) with those of the isostructuralseries {(SiP^(iPr) ₃)FeN₂}^(n). The {(CP^(iPr) ₃)FeN₂}^(n) complexesfeature an axial N₂ ligand bound trans to an axial C-atom in a trigonalbipyramidal geometry, a design meant to model one plausible geometry fora single Fe—N₂ binding site in the iron-molybdenum cofactor (FeMoco).The C_(alkyl)-Fe interaction in the (CP^(iPr) ₃)Fe system exhibits asubstantially higher degree of ionic character, and is more flexible,than for the related Si_(silyl)-Fe interaction in the isostructural andisoelectronic (SiP^(iPr) ₃)Fe system.¹⁷ These results support that thistype of Fe—C flexibility generally models the flexibility one can intuitfor an N₂—Fe—C_(interstitial) interaction within FeMoco. Whereas the N₂anion (SiP^(iPr) ₃)FeN₂ ⁻ does not effectively facilitate the deliveryof H-atoms to N₂ to produce NH₃ via proton/reductant equivalents, anEt₂O solution of (CP^(iPr) ₃)FeN₂ ⁻ under 1 atm of N₂ releases ca. 4.6equiv NH₃ relative to Fe. The modest catalytic N₂ reduction behavior of(CP^(iPr) ₃)FeN₂ ⁻ at −78° C. is comparable to (TP^(iPr)B) FeN₂ ⁻.²¹

It is noteworthy that amongst the isostructural SiP^(iPr) ₃, TP^(iPr)B,and CP^(iPr) ₃ series, the system with the most flexible axial linkage,(TP^(iPr)B)Fe, gives the greatest catalytic yield under a common set ofreaction conditions, while the least flexible, (SiP^(iPr) ₃)Fe, givesonly substoichiometric yields of ammonia; the (CP^(iPr) ₃)Fe systemfalls in between the two both in terms of flexibility and catalyticcompetence. These results may be consistent with the hypothesis that aflexible Fe—C_(interstitial) interaction might facilitate N₂ binding andreduction at a single Fe site within FeMoco. Our structural and DFTstudies²² demonstrate that, in the right environment, a carbon atom canserve as a modestly flexible ligand trans to an Fe—N₂ binding site, andthat this flexibility is enhanced by the ability of the carbon toaccommodate a significant ionic charge. It seems likely to us that theinorganic carbide ligand in FeMoco is similarly, and likely more, ableto stabilize substantial ionic character in the Fe—C_(interstitial) bond(FIG. 12), resulting in a flexible interaction that initially exposes anN₂ binding site that can be further modulated as a function of theN_(x)H_(y) reduction state.

Within our synthetic series, it may be that different catalysts followdifferent mechanistic pathways (distal vs. alternating, or some hybridpath);^(21,41) for instance the most flexible system, (TP^(iPr)B)Fe, maybe better suited to facilitate a distal pathway that samples stronglypi-bonded intermediates, while (CP^(iPr) ₃)Fe, which we presume is lessflexible, could instead be dominated by an alternating or hybridpathway.

Experimental Methods

General. All manipulations were carried out using standard Schlenk orglovebox techniques under an N₂ atmosphere. Unless otherwise noted,solvents were deoxygenated and dried by thoroughly sparging with N₂followed by passage through an activated alumina column in a solventpurification system by SG Water, USA LLC. Non-halogenated solvents weretested with a standard purple solution of sodium benzophenone ketyl intetrahydrofuran in order to confirm effective moisture removal.O-iodotriphenylmethane,²⁸ H(OEt₂)₂[B(3,5-(CF₃)₂—C₆H₃)₄],⁴² KC₈,⁴³[(TPB)FeN₂][Na(12-crown-4)₂],²⁵ [(SiP^(iPr) ₃)FeN₂][Na(12-crown-4)₂]¹⁷and [(C^(Si)P^(Ph) ₃)FeN₂][K(18-crown-6)₂]²² were prepared according toliterature procedures. [Decamethylferrocenium][B(3,5-(CF₃)₂—C₆H₃)₄] wasprepared by treating [ferrocenium][B(3,5-(CF₃)₂—C₆H₃)₄]⁴⁴ withdecamethylferrocene and used without purification. FeI₂(THF)₂ wasprepared by treating Fe powder with I₂ in THF,⁴⁵ and was dried to FeI₂by heating under vacuum at 80° C. for 6 hours. All other reagents werepurchased from commercial vendors and used without further purificationunless otherwise stated.

Physical Methods. Elemental analyses were performed by Robinson MicrolitLaboratories (Ledgewood, N.J.). Deuterated solvents were purchased fromCambridge Isotope Laboratories, Inc., degassed, and dried over active3-Å molecular sieves prior to use. ¹H and ¹³C chemical shifts arereported in ppm relative to tetramethylsilane, using residual proton and¹³C resonances from solvent as internal standards. ³¹P and ¹⁹F chemicalshifts are reported in ppm relative to 85% aqueous H₃PO₄ and CFCl₃,respectively. Solution phase magnetic measurements were performed by themethod of Evans.⁴⁶ Optical spectroscopy measurements were taken on aCary 50 UV-Vis spectrophotometer using a 1-cm two-window quartz cell.Electrochemical measurements were carried out in a glovebox under adinitrogen atmosphere in a one compartment cell using a CH Instruments600B electrochemical analyzer. A glassy carbon electrode was used as theworking electrode and platinum wire was used as the auxiliary electrode.The reference electrode was Ag/AgNO₃ in THF. The ferrocene couple Fc+/Fcwas used as an internal reference. Solutions (THF) of electrolyte (0.2 Mtetra-n-butylammonium hexafluorophosphate) and analyte were alsoprepared under an inert atmosphere.

X-ray Crystallography. XRD studies were carried out at the BeckmanInstitute Crystallography Facility on a Bruker Kappa Apex IIdiffractometer (Mo Kα radiation). Structures were solved using SHELXSand refined against F² on all data by full-matrix least squares withSHELXL.⁴⁷ The crystals were mounted on a wire loop. Methyl grouphydrogen atoms not involved in disorder were placed at calculatedpositions starting from the point of maximum electron density. All otherhydrogen atoms, except where otherwise noted, were placed atgeometrically calculated positions and refined using a riding model. Theisotropic displacement parameters of the hydrogen atoms were fixed at1.2 (1.5 for methyl groups) times the U_(eq) of the atoms to which theyare bonded.

Computations. A single-point calculation and Natural Bond Orbital (NBO)analysis was carried out on [(CP^(iPr) ₃)FeN₂][K(Et₂O)₃] (12) using thecrystallographically determined atomic coordinates at theB3LYP/6-31++G(d,p) level of theory using the Gaussion03 suite ofprograms.⁴⁸ NBO analysis located a polarized a interaction between Feand the C-atom anchor (C01).

10-phenyl-10H-dibenzo[b,e]iodininium Bromide (2). The procedure for thegeneration of 2 and 4 (below) was adapted from a reported method for thegeneration of diaryliodonium salts.²⁹ 3-chloroperoxybenzoic acid (9.0 g,˜70% by mass, ˜0.037 mol) was dissolved in dichloromethane (150 mL) andcooled to 0° C. 2-iodotriphenylmethane (11.7 g, 0.0316 mol) was added asa solid in portions over the course of 10 minutes, during which timethere was no observable change to the reaction mixture. This mixture wasstirred at 0° C. for 10 minutes and then neat trifluoromethanesulfonicacid (8.74 mL, 0.0990 mol) was added via syringe over the course of 5minutes. The reaction mixture turned dark brown. After an additional 20minutes, the reaction mixture was allowed to warm to room temperatureand stirred for one hour, and then the solvent was removed in vacuo. Thesolid material was suspended in 200 mL of diethyl ether and 200 mL ofwater, and then solid sodium bromide (14 g, 0.136 mol) was added and themixture was shaken vigorously for 5 minutes, during which time a fineoff-white precipitate developed. The precipitate was collected atop asintered glass frit and washed copiously with water and diethyl ether(14.2 g, 0.0316 mol, quant). ¹H NMR ((CD₃)₂S═O, 300 MHz, 298 K, δ): 8.27(dd, J=8 Hz, 1 Hz, 2H), 7.68 (td, J=8 Hz, 1 Hz, 2H), 7.46 (td, J=8 Hz, 1Hz), 7.27 (m, 3H), 6.78 (dm, J=8 Hz, 2H), 6.09 (s, 1H) ppm. ¹³C NMR((CD₃)₂S═O, 75.4 MHz, 298 K, δ): 140.3 (s), 138.3 (s), 135.0 (s), 132.7(s), 131.7 (s), 129.6 (s), 128.9 (s), 127.9 (s), 127.4 (s), 117.4 (s),57.7 (s) ppm. ESI-MS (positive ion, amu): Calc. 370.0; Found 370.0.

2-bromo-2′-iodotriphenylmethane (3).10-phenyl-10H-dibenzo[b,e]iodininium bromide (16.11 g, 0.0358 mol) wassuspended in dry, degassed acetonitrile (250 mL), and solidtetrabutylammonium bromide (25 g, 0.078 mol) and copper(I) bromide (8 g,0.06 mol) were added. The mixture was heated to a vigorous reflux andstirred at reflux for five days. The dark brown reaction mixture wasthen concentrated to dryness in vacuo, extracted with toluene, andfiltered through a silica plug. The pale yellow filtrate wasconcentrated to dryness and the resulting material was recrystallizedfrom methanol to give the desired product as an off-white powder whichwas collected atop a sintered glass frit and washed with cold methanol(12.7 g, 0.0282 mol, 79%). ¹H NMR (CDCl₃, 300 MHz, 298 K, δ): 7.90 (dd,J=8 Hz, 1 Hz, 1H), 7.60 (dd, J=8 Hz, 1 Hz, 1H), 7.34-7.18 (m, 5H), 7.13(td, J=8 Hz, 1 Hz, 1H), 7.03 (dd, J=8 Hz, 1 Hz, 2H), 6.95 (td, J=8 Hz, 1Hz, 1H), 6.79 (dd, J=8 Hz, 1 Hz, 2H), 6.02 (s, 1H) ppm. ¹³C NMR (CDCl₃,75.4 MHz, 298 K, δ): 145.2 (s), 142.2 (s), 141.1 (s), 140.1 (s), 133.1(s), 131.2 (s), 130.7 (s), 130.0 (s), 128.5 (s), 128.3 (s), 128.2 (s),128.0 (s), 127.2 (s), 126.7 (s), 126.3 (s), 102.9 (s), 60.8 (s) ppm. MS(amu): Calc. 449.9, 447.9; Found 449.9, 447.9.

10-(2-bromophenyl)-10H-dibenzo[b,e]iodininium iodide (4).3-chloroperoxybenzoic acid (5 g, ˜70% by mass, ˜0.0203 mol) wasdissolved in dichloromethane (200 mL) and cooled to 0° C.2-bromo-2′-iodotriphenylmethane (8.2 g, 0.0182 mol) was added as a solidin portions over the course of 10 minutes, during which time there wasno observable change in the reaction mixture. This mixture was stirredat 0° C. for 10 minutes and then neat trifluoromethanesulfonic acid(5.04 mL, 0.0571 mol) was added via syringe over the course of 5minutes. The reaction mixture turned dark brown. After an additional 30minutes, the reaction mixture was allowed to warm to room temperatureand stirred for 30 minutes, and then the solvent was removed in vacuo.The solid material was suspended in 200 mL of diethyl ether and 200 mLof water, and then solid potassium iodide (15 g, 0.090 mol) was addedand the mixture was shaken vigorously for 5 minutes, during which time afine yellow precipitate developed. The precipitate was collected atop asintered glass frit and washed copiously with water and diethyl ether(9.95 g, 0.0173 mol, 95%). ¹H NMR ((CD₃)₂S═O, 300 MHz, 298 K, δ): 8.20(dd, J=8 Hz, 1 Hz, 2H), 7.83 (dd, J=8 Hz, 1 Hz, 2H), 7.72 (dd, J=8 Hz, 1Hz, 1H), 7.60 (td, J=8 Hz, 1 Hz, 2H), 7.47-7.39 (m, 3H), 7.33 (td, J=8Hz, 1 Hz, 1H), 7.23 (dd, J=8 Hz, 1 Hz, 1H), 6.02 (s, 1H) ppm. ¹³C NMR((CD₃)₂S═O, 75.4 MHz, 298 K, δ): 138.9 (s), 135.4 (s), 135.1 (s), 135.0(s), 133.4 (s), 132.8 (s), 131.7 (s), 130.7 (s), 130.0 (s), 128.0 (s),117.2 (s), 110.0 (s), 58.8 (s) ppm. ESI-MS (positive ion, amu): Calc.446.9, 448.9; Found 446.9, 448.9.

2-bromo-2′,2″-diiodotriphenylmethane (5): Solid10-(2-bromophenyl)-10H-dibenzo[b,e]iodininium iodide (4.54 g, 7.88 mmol)was sealed inside a Schlenk tube under N₂ and heated to 200° C. for 15minutes, and then cooled to room temperature. The resulting dark violetresidue was taken up in dichloromethane (50 mL) and washed withsaturated aqueous sodium thiosulfate (50 mL) and then water (30 mL) andsaturated aqueous sodium chloride (30 mL), then dried over magnesiumsulfate, filtered, and concentrated to dryness in vacuo. The resultingoff-white residue was recrystallized from methanol to give the desiredproduct as a fine white powder, which was collected atop a sinteredglass frit and washed with cold methanol (3.4 g, 5.90 mmol, 75%). ¹H NMR(CDCl₃, 300 MHz, 298 K, δ): 7.93 (d, J=8 Hz, 2H), 7.64 (d, J=8 Hz, 1H),7.30-7.16 (m, 4H), 7.00 (t, J=8 Hz, 2H), 6.72 (d, J=8 Hz, 3H), 6.04 (s,1H) ppm. ¹³C NMR (CDCl₃, 75.4 MHz, 298 K, δ): 144.1 (s), 141.1 (s),140.2 (s), 133.3 (s), 131.1 (s), 130.7 (s), 128.6 (s), 128.5 (s), 127.3(s), 126.7 (s), 103.6 (s), 65.4 (s) ppm. MS (amu): Calc. 573.8, 575.8;Found 446.9, 448.9 ([M-I]+), 368.1 ([M-I-Br]+), 320.1, 322.1 ([M-2I]+).

Tris(2-(diisopropylphosphino)phenyl)methane (“(C^(iPr)P₃)H”) (1):2-bromo-2′,2″-diiodotriphenylmethane (2.00 g, 3.48 mmol) was dissolvedin diethyl ether (100 mL) and cooled to −78° C. while stirring. Solidt-butyllithium (1.36 g, 21.23 mmol) was added in portions over thecourse of 10 minutes and the reaction mixture was stirred at lowtemperature for 3 hours. Then chlorodiisopropylphosphine (1.96 g, 12.8mmol) was dissolved in 10 mL of diethyl ether and added to the reactionmixture. The reaction mixture was allowed to warm slowly to roomtemperature overnight, resulting in the precipitation of a fine whitesolid. The reaction mixture was filtered through silica and the paleyellow-orange filtrate was concentrated to a sticky yellow solid whichwas triturated with acetonitrile to give an off-white powder. The solidwas washed copiously with acetonitrile and then dried under vacuum,giving 1.4 g (2.36 mmol, 68%) of the desired product. ¹H NMR (C₆D₆, 300MHz, 298 K, δ): 8.15 (q, J=6 Hz, 1H), 7.44 (d, J=7 Hz, 3H), 7.06 (td,J=7 Hz, 2 Hz, 3H), 7.00-6.93 (m, 6H), 2.27 (septet of doublets, J=4 Hz,7 Hz, 3H), 1.73 (septet of doublets, J=3 Hz, 7 Hz, 3H), 1.40 (dd, J=7Hz, 13 Hz, 9H), 1.32 (dd, J=7 Hz, 12 Hz, 9H), 0.88 (dd, J=7 Hz, 13 Hz,9H), 0.44 (dd, J=7 Hz, 12 Hz, 9H) ppm. ¹³C NMR (C₆D₆, 75.4 MHz, 298 K,δ): 159.0 (d, J=29 Hz), 144.8 (d, J=17 Hz), 140.0 (s), 139.3 (s), 132.4(s), 59.1 (m), 32.7 (m), 30.0 (m), 29.4 (s), 27.3 (m), 21.0 (s) ppm. ³¹PNMR (C₆D₆, 121.4 MHz, 298 K, δ): −9.1 ppm. Anal. Calcd. for C₃₇H₅₅P₃: C,74.97; H, 9.35. Found: C, 74.73; H, 9.49.

{(CP^(iPr) ₃)H}FeI₂ (6): (CP^(iPr) ₃)H (500 mg, 0.843 mmol) was added toFeI₂ (350 mg, 1.13 mmol) in 15 mL of toluene and stirred at 60° C. for 2hours, at which point the reaction mixture was filtered through Celiteand the yellow filtrate was concentrated to give a yellow powder (761mg, 0.843 mmol, quant). Crystals suitable for X-ray diffraction weregrown by layering of pentane over a saturated toluene solution. ¹H NMR(C₆D₆, 300 MHz, 298 K, δ): 179.69, 26.00, 18.60, 14.92, 14.28, 13.62,12.74, 9.96, 9.00, 8.29, 6.76, 6.16, 5.72, 5.48, 4.97, 4.28, 3.78, 0.30,0.13, −0.48, −0.91, −2.02, −3.68, −5.09, −9.45 ppm. μ_(eff) (C₆D₆,Evans' method, 298 K): 4.85 μ_(B).

(CP^(iPr) ₃)Fe(N₂)H (9): (CP^(iPr) ₃)HFeI₂ (370 mg, 0.410 mmol) wassuspended in benzene (10 mL) and stirred vigorously over an excess of0.7% sodium/mercury amalgam (25 mg Na, 1.1 mmol) for two hours. Theinitially yellow suspension turned a deep brick red color during thistime due to the formation of {(CP^(iPr) ₃)H}FeI (7). The reactionmixture was filtered through Celite and concentrated to dryness invacuo. The deep red residue was then suspended in diethyl ether (15 mL)at −78° C. and 3 mL of dimethoxyethane was added; this solution wasvigorously stirred over excess sodium mirror for 4 hours at −78° C.,during which time the color lightened to orange. The reaction mixturewas then filtered through Celite and concentrated to dryness. Theresidue was extracted into pentane and again filtered through Celite,giving a lighter yellow-orange filtrate which was concentrated todryness again. This residue could be recrystallized from diethyl etherby slow evaporation to give yellow crystalline solids. These solids werewashed with hexamethyldisiloxane and minimal cold diethyl ether, andthen dried in vacuo to give 155 mg (0.229 mmol, 56%) of the desiredproduct. Crystals suitable for X-ray diffraction were grown byevaporation of a concentrated pentane solution intohexamethyldisiloxane. ¹H NMR (C₆D₆, 300 MHz, 298 K, δ): 7.57 (t, J=6 Hz,1H), 7.34 (m, 1H), 7.08 (m, 2H), 6.96 (m, 2H), 6.83-6.75 (m, 4H), 6.65(m, 1H), 6.50 (m, 1H), 2.94 (septet, J=8 Hz, 1H), 2.75 (m, 2H), 2.36(septet, J=6 Hz, 1H), 2.05 (septet, J=7 Hz, 1H), 1.75-1.17 (m, 25H),1.02 (dd, J=7 Hz, 11 Hz, 3H), 0.65 (dd, J=7 Hz, 15 Hz, 3H), 0.56 (dd,J=7 Hz, 10 Hz, 3H), 0.27 (dd, J=8 Hz, 13 Hz, 3H), −10.2 (ddd, J=38 Hz,53 Hz, 50 Hz) ppm. ³¹P NMR (C₆D₆, 121.4 MHz, 298 K, δ): 90.1 (dt, J=100Hz, 17 Hz, 1P), 67.0 (m, 1P), 63.4 (dt, J=100 Hz, 17 Hz, 1P) ppm. IR(thin film; cm⁻¹): 2046 (N—N), 1920 (Fe—H). Anal. Calcd. forC₃₇H₅₅FeP₃N₂: C, 65.68; H, 8.19; N, 4.14. Found: C, 65.91; H, 7.89; N,3.94.

{(CP^(iPr) ₃)H}FeBr (8): {(CP^(iPr) ₃)H}FeBr₂ (5.0 mg, 0.0070 mmol,generated by treating CP₃H with anhydrous FeBr₂ in toluene) wasdissolved in toluene, cooled to −78° C., and treated with isopropylmagnesium chloride (3.5 μL, 2.0M in Et₂O). The reaction mixture rapidlyturned dark brick-red. It was stirred at low temperature for one hourand then allowed to warm to room temperature for thirty minutes beforebeing filtered and concentrated. The dark red powder was not purified,but was analyzed by NMR in C₆D₆, and X-ray quality crystals were grownby layering pentane over a filtered benzene solution.

(CP^(iPr) ₃)FeCl (10): (CP^(iPr) ₃)Fe(N₂)H (61 mg, 0.0901 mmol) wasdissolved in diethyl ether (8 mL) and cooled to −78° C. HCl in diethylether (1.0 M, 108 μL, 0.108 mmol) was added to the solution in oneportion. The reaction mixture was stirred at low temperature for onehour and then warmed to room temperature and stirred overnight. Thecolor darkened to deep red-orange, and the reaction mixture was filteredthrough Celite and concentrated to dryness. The red residue wasrecrystallized by evaporation of a pentane solution intohexamethyldisiloxane and the resulting dark red crystals were washedsparingly with cold pentane and dried in vacuo, giving 46 mg (0.0673mmol, 75%) of (CP^(iPr) ₃)FeCl. Crystals suitable for X-ray diffractionwere grown by evaporation of a concentrated pentane solution intohexamethyldisiloxane. ¹H NMR (C₆D₆, 300 MHz, 298 K, δ): 179.93, 26.47,23.05, 17.44, 17.22, 15.03, 11.66, 1.52, −10.27, −13.36, −16.82 ppm.μ_(eff) (C₆D₆, Evans' method, 298 K): 4.92 μ_(B). Anal. Calcd. forC₃₇H₅₄FeP₃Cl: C, 65.06; H, 7.97. Found: C, 64.96; H, 8.01.

(CP^(iPr) ₃)FeN₂ (11): (CP^(iPr) ₃)FeCl (82 mg, 0.120 mmol) wasdissolved in THF (2 mL) and stirred over sodium mirror for 20 minutes,or until NMR analysis showed complete consumption of the startingmaterial, and then filtered and concentrated. The residue was extractedwith pentane and filtered through Celite, and concentrated to abrownish-orange residue which was recrystallized by evaporation of apentane solution into hexamethyldisiloxane. The dark brown-orangecrystals were washed with hexamethyldisiloxane and cold pentane anddried in vacuo to give 39 mg (0.0581 mmol, 48%) of (CP^(iPr) ₃)FeN₂.Crystals suitable for X-ray diffraction were grown by evaporation of aconcentrated pentane solution into hexamethyldisiloxane. ¹H NMR (C₆D₆,300 MHz, 298 K, δ): 19.3 (very broad), 10.4, 6.8, 3.0, 2.0, 0.6, −1.4ppm. μ_(eff) (C₆D₆, Evans' method, 298 K): 1.75 μ_(B). IR (thin film;cm⁻¹): 1992 (N—N). Anal. Calcd. for C₃₇H₅₄FeP₃N₂: C, 65.78; H, 8.06; N,4.15. Found: C, 66.03; H, 8.01; N, 3.86.

[(CP^(iPr) ₃)FeN₂][K(Et₂O)_(0.5)] (12[K(Et₂O)_(0.5)]): (CP^(iPr) ₃)FeCl(40 mg, 0.0586 mmol) was dissolved in diethyl ether (5 mL) at roomtemperature and an excess of potassium graphite (KC₈, 25 mg) was added.The reaction mixture was stirred for 10 minutes and then filteredthrough Celite. The dark brown solution was concentrated to about 2 mLand then pentane was layered over the ether solution and it was allowedto stand overnight during which time dark bluish-brown crystals formed.The supernatant was decanted and the crystals were washed thoroughlywith pentane and thoroughly dried under vacuum, giving 26 mg of thedesired product (0.0277 mmol, 47%). NMR analysis indicates the presenceof 0.5 ether solvent molecules per anion. Crystals suitable for X-raydiffraction were grown by vapor diffusion of pentane into a diethylether solution; in these crystals the potassium cation is solvated bythree diethyl ether molecules. ¹H NMR (d₈-THF, 300 MHz, 298 K, δ): 7.04(s, 3H), 6.67 (s, 3H), 6.47 (s, 6H), 3.38 (q, J=7 Hz, 2H, diethyl ether(CH₃CH₂)₂O), 2.99 (br s, 3H), 2.14 (br s, 3H), 1.42 (d, J=6 Hz, 9H),1.36 (d, J=5 Hz, 9H), 1.12 (t, J=7 Hz, 3H, diethyl ether (CH₃CH₂)₂O),1.01 (d, J=5 Hz, 9H), 0.12 (d, 9H) ppm. ³¹P NMR (5:1 C₆D₆/d₈-THF, 121.4MHz, 298 K, δ): 68.1 ppm. IR (thin film deposited from Et₂O; cm⁻¹): 1870(N—N).

[(CP^(iPr) ₃)FeN₂][K(12-c-4)₂] (12[K(12-c-4)₂]). A sample of 12 (15 mg,0.020 mmol) was dissolved in diethyl ether (1 mL) and 12-crown-4 (8.8mg, 0.050 mmol) was added as a solution in diethyl ether (1 mL). Theresulting solution was layered with pentane and allowed to standovernight, resulting in the crystallization of 12[K(12-crown-4)₂] as avery dark blue solid. The crystals were washed with pentane and driedunder vacuum, giving 10 mg of material (53% yield). ¹H NMR (d₈-THF, 300MHz, 298 K, δ) 6.86 (br s, 6H), 6.47 (s, 6H), 3.62 (s, 36H, 12-crown-4),1.43 (s, 9H), 1.30 (s, 9H), 0.91 (s, 9H), 0.16 (s, 9H) ppm. ³¹P (C₆D₆,121.4 MHz, 298 K, δ): 66 ppm. IR (thin film; cm⁻¹) 1905 (N—N).

[(CP^(iPr) ₃)FeN₂][B(3,5-(CF₃)₂—C₆H₃)₄] (13): (C^(iPr)P₃)FeN₂ (7.3 mg)was dissolved in diethyl ether (1 mL) and a solution of[Fe(C₅Me₅)₂][B(3,5-(CF₃)₂—C₆H₃)₄] in diethyl ether (1 mL) was addeddropwise while stirring at room temperature. The reaction mixture wasthen concentrated to give an orange solid which was washed with benzeneand then dried in vacuo. Crystals suitable for X-ray diffraction weregrown by slow evaporation of a diethyl ether solution intohexamethyldisiloxane. ¹H NMR (4:1 C₆D₆/THF-d₈ under N₂, 300 MHz, 298 K,δ): 16.65, 14.48, 8.15, 7.60, 2.71 ppm. (Note: the exact position of theparamagnetically shifted NMR peaks varies with the composition of thesolvent due to the likely exchange of the N₂ ligand with THF). μ_(eff)(d₈-THF, Evans' method, 298 K): 4.3 μ_(B). IR (thin film; cm⁻¹): 2128(N—N). Satisfactory elemental analysis could not be obtained due to thelability of the coordinated N₂ ligand.

(CP^(iPr) ₃)FeN₂SiMe₃ (14): [(CP^(iPr) ₃)FeN₂][K(Et₂O)_(0.5)] (35 mg,0.0465 mmol) was dissolved in diethyl ether (2 mL) and cooled to −78° C.Trimethylsilyl chloride (6 μL, 0.0473 mmol) was dissolved in diethylether (1 mL) and added dropwise to the stirring reaction mixture. Thereaction was stirred at low temperature for one hour and then warmed toroom temperature for one hour, concentrated to dryness, taken up inpentane, filtered through Celite, and concentrated. The red-orangeresidue was recrystallized by slow evaporation of a pentane solutioninto hexamethyldisiloxane, and the resulting red solids were washed withcold hexamethyldisiloxane and dried in vacuo to give 21 mg (0.0280 mmol,60%) of solid material, which was contaminated with a small amount ofCP₃FeN₂ (11) which we were unable to remove by repeatedrecrystallization. 14 decomposes slowly to 11 over time. ¹H NMR (C₆D₆,300 MHz, 298 K, δ) 7.33 (br m, 3H), 6.80 (t, J=4 Hz, 6H), 6.63 (m, 3H),2.67 (septet, J=7 Hz, 3H), 1.97 (septet, J=7 Hz, 3H), 1.45 (m, 18H),0.96 (q, J=7 Hz, 9H), 0.72 (q, J=7 Hz, 9H), 0.12 (s, 3H) ppm. ³¹P (C₆D₆,121.4 MHz, 298 K, δ): 80.1 ppm. IR (thin film; cm⁻¹) 1736 (N—N).

Ammonia Quantification. A Schlenk tube was charged with HCl (4 mL of a1.0 M solution in Et₂O, 4 mmol). Reaction mixtures were vacuumtransferred into this collection flask. Residual solid in the reactionvessel was treated with a solution of [Na][O-t-Bu] (40 mg, 0.4 mmol) in1,2-dimethoxyethane (1 mL) and sealed. The resulting suspension wasallowed to stir for 10 minutes before all volatiles were again vacuumtransferred into the collection flask. After completion of the vacuumtransfer, the flask was sealed and warmed to room temperature. Solventwas removed in vacuo and the remaining residue was dissolved in H₂O (1mL). An aliquot of this solution (20 μL) was then analyzed for thepresence of NH₃ (trapped as [NH₄][Cl]) via the indophenol method.³⁸Quantification was performed with UV-Vis spectroscopy by analyzingabsorbance at 635 nm.

Standard catalytic procedure with [(CP^(iPr) ₃)FeN₂][K(Et₂O)_(0.5)](12): [(CP^(iPr) ₃)FeN₂][K(Et₂O)_(0.5)] (1.9 mg, 0.0025 mmol) wasdissolved in Et₂O (0.5 mL) in a small Schlenk tube equipped with a stirbar. This solution was cooled to −78° C. in a cold well inside of theglove box. A suspension of KC₈ (14 mg, 0.100 mmol) in Et₂O (0.75 mL) wascooled to −78° C. and added to the reaction mixture with stirring. Afterfive minutes, a similarly cooled solution of HBAr^(F) ₄.2 Et₂O (93 mg,0.092 mmol) in Et₂O (1.0 mL) was added to the suspension in one portionwith rapid stirring. Any remaining acid was dissolved in cold Et₂O (0.25mL) and added subsequently, and the Schlenk tube was sealed. Thereaction was allowed to stir for 60 minutes at −78° C. before beingwarmed to room temperature and stirred for 15 minutes.

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Synthesis of o-nitrotriphenylmethane: (Note: The synthesis of thisspecies, a precursor to o-iodotriphenylmethane, has been reported;¹ herewe report the synthesis again in order to detail slight modifications tothe workup and purification procedures that facilitate its synthesis.The syntheses of o-aminotriphenylmethane and o-iodotriphenylmethane havenot been substantially modified from the reported procedures.) Aluminumchloride (30.36 g, 0.228 mol) was suspended in 100 mL of benzene ando-nitrobenzaldehyde (15 g, 0.099 mol) was added as a solid in portionsover 15 minutes while stirring at room temperature. The reaction mixturewas heated to reflux for 6 hours, then cooled to room temperature andpoured over ice (300 mL). The mixture was diluted with another 100 mL ofbenzene and 100 mL of water. The aqueous layer was removed and washedtwice with benzene (100 mL); the dark brown organic washings were thencombined and washed repeatedly with concentrated H₂SO₄ until the colorwas light yellow-orange. The organic layer was then washed with water(100 mL) and brine (2×50 mL), dried over magnesium sulfate, filtered,and concentrated. The off-white oily residue was recrystallized frommethanol to give a white crystalline solid which was collected atop aglass frit and washed with cold methanol (15.6 g, 54%). ¹H NMR (CDCl₃,300 MHz, 298 K, δ): 7.89 (d, J=8 Hz, 1H), 7.49 (t, J=8 Hz, 1H), 7.39 (t,J=8 Hz, 1H), 7.34-7.23 (m, 6H), 7.14-7.08 (m, 4H) ppm. ¹³C NMR (CDCl₃,75.4 MHz, 298 K, δ): 149.77 (s), 141.95 (s), 138.13 (s), 132.41 (s),132.04 (s), 129.51 (s), 128.54 (s), 127.49 (s), 126.85 (s), 124.76 (s)ppm.

Catalytic Production of NH₃ using [(CP^(iPr) ₃)FeN₂][K(Et₂O)_(0.5)]

Standard catalytic procedure with [(CP^(iPr) ₃)FeN₂][K(Et₂O)_(0.5)](12): [(CP^(iPr) ₃)FeN₂][K(Et₂O)_(0.5)] (1.9 mg, 0.0025 mmol) wassuspended in Et₂O (0.5 mL) in a small Schlenk tube equipped with a stirbar. This solution was cooled to −78° C. in a cold well inside of theglove box. A suspension of KC₈ (14 mg, 0.100 mmol) in Et₂O (0.75 mL) wascooled to −78° C. and added to the reaction mixture with stirring. Afterfive minutes, a similarly cooled solution of HBAr^(F) ₄.2 Et₂O (93 mg,0.092 mmol) in Et₂O (1.0 mL) was added to the suspension in one portionwith rapid stirring. Any remaining acid was dissolved in cold Et₂O (0.25mL) and added subsequently, and the Schlenk tube was sealed. Thereaction was allowed to stir for 60 minutes at −78° C. before beingwarmed to room temperature and stirred for 15 minutes.

Results of Individual Runs Run Absorbance Eq. NH₃/Fe % Yield based on H+A¹ 0.446 3.6 28 B 0.466 4.3 34 C 0.655 6.0 47 D 0.476 4.4 35 E 0.491 4.536 F 0.451 4.2 33 G 0.472 4.3 34 H 0.587 5.4 43 Avg 0.514 ± 0.08 4.6 ±0.8 36 ± 6 ¹Used 2.2 mg (.0029 mmol) of catalyst; omitted from averageabsorbance.

Hydrazine was not detected in the catalytic runs using a standard UV-Visquantification method².

Modified “acid-first” catalytic procedure with [(CP^(iPr)₃)FeN₂][K(Et₂O)_(0.5)](12): [(CP^(iPr) ₃)FeN₂][K(Et₂O)_(0.5)] (1.9 mg,0.0025 mmol) was dissolved in Et₂O (0.5 mL) in a 20 mL scintillationvial equipped with a stir bar. This dark brown solution was vigorouslystirred and cooled to −78° C. in a cold well inside of the glove box. Asimilarly cooled solution of HBAr^(F) ₄.2 Et₂O (93 mg, 0.092 mmol) inEt₂O (1.5 mL) was added to the solution in one portion with rapidstirring. Any remaining acid was dissolved in cold Et₂O (0.25 mL) andadded subsequently. The reaction mixture turned light orange uponaddition of acid and the resulting solution was allowed to stir for 5minutes before being transferred into a pre-cooled Schlenk tube equippedwith a stirbar. The original reaction vial was washed with cold Et₂O(0.25 mL) which was subsequently transferred to the Schlenk tube. SolidKC₈ (14 mg, 0.100 mmol) was suspended in cold Et₂O (0.75 mL) and addeddropwise to the rapidly stirred solution in the Schlenk tube which wasthen tightly sealed. The reaction was allowed to stir for 60 minutes at−78° C. before being warmed to room temperature and stirred for 15minutes.

Results of individual runs Run Absorbance Eq. NH₃/Fe % Yield based on H₂I¹ 0.375 3.0 24 J 0.483 4.3 34 K 0.484 4.4 35 M 0.407 3.8 30 Avg. 0.458± 0.04 3.8 ± 0.6 30 ± 5 ¹Used 2.2 mg (.0029 mmol) of catalyst; omittedfrom average absorbance.

Standard Catalytic Procedure with [(TPB)FeN₂][Na(12-c-4)₂]:[(TPB)FeN₂][Na(12-c-4)₂] (2.0 mg, 0.002 mmol) was suspended in Et₂O (0.5mL) in a small Schlenk tube equipped with a stir bar. This solution wascooled to −78° C. in a cold well inside of the glove box. A suspensionof KC₈ (14 mg, 0.100 mmol) in Et₂O (0.75 mL) was cooled to −78° C. andadded to the reaction mixture with stirring. After five minutes, asimilarly cooled solution of HBAr^(F) ₄.2 Et₂O (93 mg, 0.092 mmol) inEt₂O (1.0 mL) was added to the suspension in one portion with rapidstirring. Any remaining acid was dissolved in cold Et₂O (0.25 mL) andadded subsequently, and the Schlenk tube was sealed. The reaction wasallowed to stir for 60 minutes at −78° C. before being warmed to roomtemperature and stirred for 15 minutes.

Results of Individual Runs Run Absorbance Eq. NH₃/Fe % Yield based on H+N 0.528 6.1 38 O 0.422 3.9 24 Avg. 0.475 ± 0.05 5.0 ± 1.1 31 ± 7

Standard catalytic procedure with [(CP^(iPr) ₃)FeN₂][K(12-c-4)₂](12[K(12-crown-4)₂]): [(CP^(iPr) ₃)FeN₂][K(12-c-4)₂] (2.0 mg, 0.002mmol) was suspended in Et₂O (0.5 mL) in a small Schlenk tube equippedwith a stir bar. This solution was cooled to −78° C. in a cold wellinside of the glove box. A suspension of KC₈ (14 mg, 0.100 mmol) in Et₂O(0.75 mL) was cooled to −78° C. and added to the reaction mixture withstirring. After five minutes, a similarly cooled solution of HBAr^(F)₄.2 Et₂O (93 mg, 0.092 mmol) in Et₂O (1.0 mL) was added to thesuspension in one portion with rapid stirring. Any remaining acid wasdissolved in cold Et₂O (0.25 mL) and added subsequently, and the Schlenktube was sealed. The reaction was allowed to stir for 60 minutes at −78°C. before being warmed to room temperature and stirred for 15 minutes.

Results of Individual Runs Run Absorbance Eq. NH₃/Fe % Yield based on H+P 0.327 3.7 23 Q 0.328 3.7 23 R¹ 0.344 3.1 24 Avg. 0.328 ± 0.001 3.5 ±0.3 23 ± 0.3 ¹Used 2.5 mg (.0025 mmol) of catalyst; omitted from averageabsorbance.

Standard catalytic procedure with [(SiP^(iPr) ₃)FeN₂][Na(12-c-4)₂]:[(SiP^(iPr) ₃)FeN₂][Na(12-c-4)₂] (2.0 mg, 0.002 mmol) was suspended inEt₂O (0.5 mL) in a small Schlenk tube equipped with a stir bar. Thissolution was cooled to −78° C. in a cold well inside of the glove box. Asuspension of KC₈ (14 mg, 0.100 mmol) in Et₂O (0.75 mL) was cooled to−78° C. and added to the reaction mixture with stirring. After fiveminutes, a similarly cooled solution of HBAr^(F) ₄.2 Et₂O (93 mg, 0.092mmol) in Et₂O (1.0 mL) was added to the suspension in one portion withrapid stirring. Any remaining acid was dissolved in cold Et₂O (0.25 mL)and added subsequently, and the Schlenk tube was sealed. The reactionwas allowed to stir for 60 minutes at −78° C. before being warmed toroom temperature and stirred for 15 minutes.

Results of Individual Runs Run Absorbance Eq. NH₃/Fe % Yield based on H+S 0.109 1.2 8 T 0.040 0.4 3 Avg. 0.075 ± 0.03 0.8 ± 0.4 5 ± 3

Identification of H₂ in a standard catalytic run. A catalytic run wasperformed with 0.0025 mmol of 12 according to the standard procedure.Prior to the vacuum transfer of volatiles, the solutions inside of theSchlenk tubes were frozen. The ground glass joint of the Schlenk tubewas then sealed with a rubber septum and the head space between theTeflon stopcock of the Schlenk tube and the septum was evacuated. Thishead space was left under static vacuum and the Teflon stopcock of thereaction vessel was opened after which a 10 mL aliquot of the headspacewas sampled through the septum via a gas-tight syringe. This sample wasthen analyzed for hydrogen with an Agilent 7890A gas chromatograph usinga thermal conductivity detector. 45% yield of H₂ relative to H⁺ wasquantified.

IR spectral analysis of addition of 12 equiv. of KC₈, followed by 10equiv of HBAr^(F) ₄.2 Et₂O to [(CP^(iPr) ₃)FeN₂][K(Et₂O)_(0.5)]. A 20 mLscintillation vial was charged with a stir bar and [(CP^(iPr)₃)FeN₂][K(Et₂O)_(0.5)] (7.6 mg, 0.0101 mmol). In a separate vial,HBAr^(F) ₄.2 Et₂O (75 mg, 0.074 mmol) was dissolved in Et₂O (1 mL).Finally, a third vial was prepared containing a suspension of potassiumgraphite (12 mg, 0.090 mmol) in Et₂O (1 mL). All three vials werechilled in the cold well to −78° C. for 10 minutes. The suspension ofKC₈ was quickly added to the stirring suspension of [(CP^(iPr)₃)FeN₂][K(Et₂O)_(0.5)]. After stirring for 5 minutes, HBAr^(F) ₄.2 Et₂Owas added rapidly to the stirring reaction mixture. This solution wascapped and stirred at −78° C. for 60 minutes and then brought to r.t.and stirred for 15 minutes. The resulting reaction mixture wasconcentrated to dryness, taken up in C₆D₆, and filtered through Celite,giving an orange solution which was analyzed by IR and NMR. By IR, themajor species appeared to be (CP^(iPr) ₃)FeN₂ (11), and the presence ofa smaller amount of (CP^(iPr) ₃)Fe(N₂)(H) (9) was also apparent. By NMR,11, 9, and (CP^(iPr) ₃)FeCl (10) were detected as well as an additionalunidentified diamagnetic species present in small amounts. Nouncoordinated (CP^(iPr) ₃)H ligand could be detected.

IR spectral analysis of addition of 40 equiv. of KC₈, followed by 38equiv of HBAr^(F) ₄.2 Et₂O to [(CP^(iPr) ₃)FeN₂][K(Et₂O)_(0.5)]. A 20 mLscintillation vial was charged with a stir bar and [(CP^(iPr)₃)FeN₂][K(Et₂O)_(0.5)] (8.0 mg, 0.0106 mmol). In a separate vial,HBAr^(F) ₄.2 Et₂O (392 mg, 0.403 mmol) was dissolved in Et₂O (1 mL).Finally, a third vial was prepared containing a suspension of potassiumgraphite (59 mg, 0.424 mmol) in Et₂O (1 mL). All three vials werechilled in the cold well to −78° C. for 10 minutes. The suspension ofKC₈ was quickly added to the stirring suspension of [(CP^(iPr)₃)FeN₂][K(Et₂O)_(0.5)]. After stirring for 5 minutes, HBAr^(F) ₄.2 Et₂Owas added rapidly to the stirring reaction mixture. This solution wascapped and stirred at −78° C. for 60 minutes and then brought to r.t.and stirred for 15 minutes. The resulting reaction mixture wasconcentrated to dryness, taken up in C₆D₆, and filtered through Celite,giving an orange solution which was analyzed by IR and NMR. By IR andNMR, the major species appeared to be (CP^(iPr) ₃)Fe(N₂)(H) (9), andsome (CP^(iPr) ₃)FeN₂ (11) was also present. No uncoordinated (CP^(iPr)₃)H ligand could be detected.

In a separate experiment, the reaction was carried out as above, butafter being allowed to stir at room temperature for 15 minutes thereaction mixture was filtered and to the filtrate was added an aliquotof a standard solution of 1,3,5-trimethoxybenzene (0.0106 mmol). Thecombined solution was concentrated to dryness, taken up in C₆D₆,filtered through Celite, and analyzed by IR and NMR. NMR integration(d1=10 sec) of the diamagnetic peaks shows approximately 70% yield of 9relative to the starting catalyst (FIG. 49).

Curves were generated by creating solutions of [NH₄][Cl] and[N₂H₅][HSO₄] of known concentrations and then analyzing by theappropriate UV-Vis methodology (vide supra)

Quantification of ammonia formed without added reductant. A sample of12[K(Et₂O)_(0.5)] was dissolved in Et₂O (1 mL) and cooled to −78° C. ina Schlenk tube. HBAr^(F) ₄.2 Et₂O (10 equiv.) was dissolved in cold Et₂O(1 mL) and added in one portion with rapid stirring. The reactionmixture was stirred at −78° C. for 1 hour and then at room temperaturefor 20 minutes, and then subjected to the standard ammoniaquantification procedure. One run using 0.0025 mmol of 12[K(Et₂O)_(0.5)]gave <0.05 equiv. of NH₃ (below detection limits) while a second runusing 0.005 mmol of 12[K(Et₂O)_(0.5)] gave 0.06 equiv. of NH₃/Fe.

Attempted catalysis with 9. (CP₃ ^(iPr))Fe(N₂)(H) (1.7 mg, 0.002 mmol)was suspended in Et₂O (0.5 mL) in a small Schlenk tube equipped with astir bar. This solution was cooled to −78° C. in a cold well inside ofthe glove box. A suspension of KC₈ (14 mg, 0.100 mmol) in Et₂O (0.75 mL)was cooled to −78° C. and added to the reaction mixture with stirring.After five minutes, a similarly cooled solution of HBAr^(F) ₄.2 Et₂O (93mg, 0.092 mmol) in Et₂O (1.0 mL) was added to the suspension in oneportion with rapid stirring. Any remaining acid was dissolved in coldEt₂O (0.25 mL) and added subsequently, and the Schlenk tube was sealed.The reaction was allowed to stir for 60 minutes at −78° C. before beingwarmed to room temperature and stirred for 15 minutes. The reaction wassubjected to the standard workup and ammonia detection procedure; noammonia was detected.

Attempted catalysis with [(C^(Si)P^(Ph) ₃)FeN₂][K(benzo-15-crown-5)₂]:[(C^(Si)P^(Ph) ₃)FeN₂][K(benzo-15-crown-5)₂] (2.0 mg, 0.0020 mmol) wasdissolved in Et₂O (0.5 mL) in a 20 mL scintillation vial equipped with astir bar. This dark brown solution was vigorously stirred and cooled to−78° C. in a cold well inside of the glove box. A similarly cooledsolution of HBAr^(F) ₄.2 Et₂O (93 mg, 0.092 mmol) in Et₂O (1.5 mL) wasadded to the solution in one portion with rapid stirring. Any remainingacid was dissolved in cold Et₂O (0.25 mL) and added subsequently. Thereaction mixture was allowed to stir for 5 minutes before beingtransferred into a pre-cooled Schlenk tube equipped with a stirbar. Theoriginal reaction vial was washed with cold Et₂O (0.25 mL) which wassubsequently transferred to the Schlenk tube. Solid KC₈ (14 mg, 0.100mmol) was suspended in cold Et₂O (0.75 mL) and added dropwise to therapidly stirred solution in the Schlenk tube which was then tightlysealed. The reaction was allowed to stir for 60 minutes at −78° C.before being warmed to room temperature and stirred for 15 minutes.Ammonia was quantified via the standard method. Two trials wereperformed, giving 0.40 and 0.14 equiv. NH₃/Fe.

Crystal Structure Tables and Refinement Information

Crystal data and structure refinement for [(CP^(iPr) ₃)H]Fel₂ (6)Empirical formula C40.50 H59 Fe I2 P3 Formula weight 948.44 Temperature296(2) K Wavelength 0.71073 Å Crystal system Monoclinic Space group C2/cUnit cell dimensions a = 39.338(2) Å α = 90°. b = 12.1860(7) Å β =111.327(2)°. c = 18.6670(11) Å γ = 90°. Volume 8335.6(8) Å³ Z 8 Density(calculated) 1.512 Mg/m³ Absorption coefficient 1.985 mm⁻¹ F(000) 3832Theta range for data collection 1.76 to 26.37°. Index ranges −49 <= h <=49, −15 <= k <= 15, −23 <= l <= 23 Reflections collected 117932Independent reflections 8536 [R(int) = 0.0714] Completeness to theta =26.37° 100.0% Absorption correction Semi-empirical from equivalents Max.and min. transmission .7460 and .6679 Refinement method Full-matrixleast-squares on F² Data/restraints/parameters 8536/464/433Goodness-of-fit on F² 1.037 Final R indices [I>2sigma(I)] R1 = 0.0256,wR2 = 0.0462 R indices (all data) R1 = 0.0397, wR2 = 0.0501 Largestdiff. peak and hole 0.491 and −0.498 e.Å⁻³

One solvent molecule is present in the structure of 6, a toluenemolecule which was modeled as disordered over a two-fold specialposition. Additionally, one isopropyl group on the ligand was refined asa disorder over two positions in a 76:24 ratio.

Crystal data and structure refinement for [(CP^(iPr) ₃)H]FeBr (8)Empirical formula C43 H61 Br Fe P3 Formula weight 814.48 Temperature100(2) K Wavelength 0.71073 Å Crystal system Monoclinic Space groupP2(1)/c Unit cell dimensions a = 12.5154(8) Å α = 90°. b = 15.3220(10) Åβ = 92.849(3)°. c = 20.9097(10) Å γ = 90°. Volume 4004.7(4) Å³ Z 4Density (calculated) 1.351 Mg/m³ Absorption coefficient 1.525 mm⁻¹F(000) 1684 Crystal size 0.30 × 0.06 × 0.02 mm³ Theta range for datacollection 1.63 to 27.89°. Index ranges −16 <= h <= 16, −19 <= k <= 19,−27 <= l <= 26 Reflections collected 61095 Independent reflections 9120[R(int) = 0.1687] Completeness to theta = 25.00° 100.0% Absorptioncorrection Semi-empirical from equivalents Max. and min. transmission0.9701 and 0.6576 Refinement method Full-matrix least-squares on F²Data/restraints/parameters 9120/0/427 Goodness-of-fit on F² 0.970 FinalR indices [I>2sigma(I)] R1 = 0.0618, wR2 = 0.1127 R indices (all data)R1 = 0.1742, wR2 = 0.1545 Largest diff. peak and hole 1.110 and −0.862e.Å⁻³

The structure of 8 includes one benzene molecule per asymmetric unit.

Crystal data and structure refinement for (CP^(iPr) ₃)Fe(N₂)(H) (9)Empirical formula C37 H55 Fe N2 P3 Formula weight 676.59 Temperature296(2) K Wavelength 0.71073 Å Crystal system Orthorhombic Space groupP2(1)2(1)2(1) Unit cell dimensions a = 10.8051(3) Å α = 90°. b =15.4905(5) Å β = 90°. c = 20.7380(7) Å γ = 90°. Volume 3471.05(19) Å³ Z4 Density (calculated) 1.295 Mg/m³ Absorption coefficient 0.601 mm⁻¹F(000) 1448 Crystal size 0.50 × 0.49 × 0.38 mm³ Theta range for datacollection 1.96 to 25.00°. Index ranges −12 <= h <= 12, −18 <= k <= 18,−24 <= l <= 24 Reflections collected 65697 Independent reflections 6113[R(int) = 0.0393] Completeness to theta = 25.00° 99.9% Absorptioncorrection Semi-empirical from equivalents Max. and min. transmission0.8037 and 0.7530 Refinement method Full-matrix least-squares on F²Data/restraints/parameters 6113/341/398 Goodness-of-fit on F² 1.059Final R indices [I>2sigma(I)] R1 = 0.0304, wR2 = 0.0776 R indices (alldata) R1 = 0.0306, wR2 = 0.0777 Absolute structure parameter 0.099(12)Largest diff. peak and hole 1.783 and −0.639 e.Å⁻³

Crystal data and structure refinement for (CP^(iPr) ₃)FeN₂ (11)Empirical formula C37 H54 Fe N2 P3 Formula weight 675.58 Temperature100(2) K Wavelength 0.71073 Å Crystal system Trigonal Space group R-3Unit cell dimensions a = 19.4069(5) Å α = 90°. b = 19.4069(5) Å β = 90°.c = 47.9751(17) Å γ = 120°. Volume 15648.0(8) Å³ Z 18 Density(calculated) 1.290 Mg/m³ Absorption coefficient 0.600 mm⁻¹ F(000) 6498Theta range for data collection 1.27 to 25.03°. Index ranges −23 <= h <=11, −23 <= k <= 23, −57 <= l <= 57 Reflections collected 12326Independent reflections 12326 [R(int) = 0.0000] Completeness to theta =25.03° 100.0% Absorption correction Semi-empirical from equivalentsRefinement method Full-matrix least-squares on F²Data/restraints/parameters 12326/335/389 Goodness-of-fit on F² 1.106Final R indices [I>2sigma(I)] R1 = 0.0624, wR2 = 0.1624 R indices (alldata) R1 = 0.0655, wR2 = 0.1672 Largest diff. peak and hole 2.593 and−0.891 e.Å⁻³

The refinement of the crystal structure of 11 used SHELX HKLF 5refinement to treat the presence of a non-merohedral twin accounting for20% of the observed reflections.

Crystal data and structure refinement for [(CP^(iPr) ₃)FeN₂][K(Et₂O)₃](12) Empirical formula C49 H84 Fe K N2 O3 P3 Formula weight 937.04Temperature 296(2) K Wavelength 0.71073 Å Crystal system MonoclinicSpace group Cc Unit cell dimensions a = 10.9229(4) Å α = 90°. b =26.9376(10) Å β = 94.663(2)°. c = 17.6054(7) Å γ = 90°. Volume 5163.0(3)Å³ Z 4 Absorption coefficient 0.505 mm⁻¹ F(000) 2024 Theta range fordata collection 1.91 to 35.46°. Index ranges −17 <= h <= 17, −38 <=k <=43, −28 <= l <= 28 Reflections collected 77464 Independent reflections21119 [R(int) = 0.0624] Completeness to theta = 35.46° 94.5% Absorptioncorrection Semi-empirical from equivalents Max. and min. transmission0.9701 and 0.6576 Refinement method Full-matrix least-squares on F²Data/restraints/parameters 21119/2/544 Goodness-of-fit on F² 1.008 FinalR indices [I>2sigma(I)] R1 = 0.0438, wR2 = 0.0843 R indices (all data)R1 = 0.0755, wR2 = 0.0949 Absolute structure parameter 0.130(8) Largestdiff. peak and hole 0.902 and −0.524 e.Å⁻³

Crystal data and structure refinement for [(CP^(iPr) ₃)FeN₂][BAr₄ ^(F)](13) Empirical formula C69 H66 B Cl0.12 F24 Fe N1.77 P3 Formula weight1539.84 Temperature 296(2) K Wavelength 0.71073 Å Crystal systemOrthorhombic Space group Pbca Unit cell dimensions a = 19.7846(11) Å α =90°. b = 25.8800(14) Å β = 90°. c = 26.6463(13) Å γ = 90°. Volume13643.6(13) Å³ Z 8 Density (calculated) 1.499 Mg/m³ Absorptioncoefficient 0.405 mm⁻¹ F(000) 6291 Theta range for data collection 1.50to 30.53°. Index ranges −28 <= h <= 28, −36 <= k <= 36, −38 <= l <= 36Reflections collected 224578 Independent reflections 20858 [R(int) =0.1311] Completeness to theta = 30.53° 99.9% Absorption correctionSemi-empirical from equivalents Max. and min. transmission 0.9701 and0.6576 Refinement method Full-matrix least-squares on F²Data/restraints/parameters 20858/1048/946 Goodness-of-fit on F² 1.118Final R indices [I>2sigma(I)] R1 = 0.0845, wR2 = 0.2235 R indices (alldata) R1 = 0.1320, wR2 = 0.2520 Largest diff. peak and hole 1.915 and−1.004 e.Å⁻³

This crystal structure was modeled with a 12% occupancy of the chloridecomplex, CP₃FeCl. Additionally, one isopropyl group of the CP₃ ligandwas modeled as disordered over two positions in a 49:51 ratio, and onetrifluoromethyl group of the BArF anion was modeled as disordered overtwo positions in a 48:52 ratio.

Tables of Selected Bond Lengths and Angles

Selected bond lengths [Å] and angles [°] for [(CP^(iPr) ₃)H]Fel₂ (6):I(2)—Fe(1) 2.5884(4) I(1)—Fe(1) 2.6062(4) Fe(1)—P(2) 2.4587(7)Fe(1)—P(1) 2.5134(7) P(2)—Fe(1)—P(1) 113.21(2) P(2)—Fe(1)—I(2)109.349(19) P(1)—Fe(1)—I(2) 116.625(19) P(2)—Fe(1)—I(1) 102.335(19)P(1)—Fe(1)—I(1) 99.721(18) I(2)—Fe(1)—I(1) 114.490(14)

Selected bond lengths [Å] and angles [°] for [(CP^(iPr) ₃)H]FeBr (8)Br(1)—Fe(1) 2.4647(7) Fe(1)—P(1) 2.3759(15) Fe(1)—P(2) 2.3940(15)Fe(1)—P(3) 2.3968(15) P(1)—Fe(1)—P(2) 110.58(5) P(1)—Fe(1)—P(3)109.39(5) P(2)—Fe(1)—P(3) 110.12(5) P(1)—Fe(1)—Br(1) 109.39(4)P(2)—Fe(1)—Br(1) 107.70(4) P(3)—Fe(1)—Br(1) 109.63(4)

Selected bond lengths [Å] and angles [°] for (CP^(iPr) ₃)Fe(N₂)(H) (9).Fe(1)—N(1) 1.8144(19) Fe(1)—C(0) 2.155(2) Fe(1)—P(2) 2.1857(7)Fe(1)—P(3) 2.2502(7) Fe(1)—P(1) 2.2779(7) Fe(1)—H(01) 1.45(3) N(1)—N(2)1.106(3) N(1)—Fe(1)—C(0) 177.27(9) N(1)—Fe(1)—P(2) 95.87(7)C(0)—Fe(1)—P(2) 86.48(6) N(1)—Fe(1)—P(3) 94.18(7) C(0)—Fe(1)—P(3)83.11(6) P(2)—Fe(1)—P(3) 141.23(3) N(1)—Fe(1)—P(1) 97.38(7)C(0)—Fe(1)—P(1) 83.41(6) P(2)—Fe(1)—P(1) 103.08(3) P(3)—Fe(1)—P(1)112.60(2) N(1)—Fe(1)—H(01) 95.5(13) C(0)—Fe(1)—H(01) 83.9(13)P(2)—Fe(1)—H(01) 72.5(13) P(3)—Fe(1)—H(01) 69.3(13) P(1)—Fe(1)—H(01)166.8(13)

Selected bond lengths [Å] and angles [°] for (CP^(iPr) ₃)FeN₂ (11)Fe(1)—N(1) 1.797(2) Fe(1)—C(01) 2.152(3) Fe(1)—P(1) 2.2349(8) Fe(1)—P(3)2.2602(8) Fe(1)—P(2) 2.2660(8) N(1)—N(2) 1.134(4) N(1)—Fe(1)—C(01)174.80(11) N(1)—Fe(1)—P(1) 96.71(8) C(01)—Fe(1)—P(1) 82.62(6)N(1)—Fe(1)—P(3) 103.51(9) C(01)—Fe(1)—P(3) 81.50(7) P(1)—Fe(1)—P(3)110.81(3) N(1)—Fe(1)—P(2) 94.67(9) C(01)—Fe(1)—P(2) 82.11(6)P(1)—Fe(1)—P(2) 132.54(3) P(3)—Fe(1)—P(2) 110.90(3)

Selected bond lengths [Å] and angles[°] for [(CP^(iPr) ₃)FeN₂][K(Et₂O)₃]Fe(1)—N(1) 1.7397(16) Fe(1)—C(01) 2.1646(17) Fe(1)—P(3) 2.1947(5)Fe(1)—P(2) 2.2045(5) Fe(1)—P(1) 2.2047(5) K(1)—N(2) 2.6560(18) N(1)—N(2)1.153(2) N(1)—Fe(1)—C(01) 179.68(7) N(1)—Fe(1)—P(3) 96.27(5)C(01)—Fe(1)—P(3) 83.81(5) N(1)—Fe(1)—P(2) 96.29(5) C(01)—Fe(1)—P(2)83.93(5) P(3)—Fe(1)—P(2) 120.051(19) N(1)—Fe(1)—P(1) 96.07(5)C(01)—Fe(1)—P(1) 83.62(5) P(3)—Fe(1)—P(1) 118.686(19) P(2)—Fe(1)—P(1)117.797(19) N(2)—N(1)—Fe(1) 179.86(19) N(1)—N(2)—K(1) 155.77(15)

Selected bond lengths [Å] and angles [°] for [(CP^(iPr) ₃)FeN₂][BAr₄^(F)] (13) Fe(1)—N(1) 1.864(7) Fe(1)—C(01) 2.081(3) Fe(1)—P(1) 2.3248(9)Fe(1)—P(3) 2.3604(9) Fe(1)—P(2) 2.3885(10) N(1)—Fe(1)—C(01) 177.7(2)N(1)—Fe(1)—P(1) 93.62(18) C(01)—Fe(1)—P(1) 84.31(8) N(1)—Fe(1)—P(3)96.5(2) C(01)—Fe(1)—P(3) 85.20(8) P(1)—Fe(1)—P(3) 117.04(3)N(1)—Fe(1)—P(2) 95.8(2) C(01)—Fe(1)—P(2) 84.67(8) P(1)—Fe(1)—P(2)124.77(4) P(3)—Fe(1)—P(2) 115.66(3) N(2)—N(1)—Fe(1) 178.4(6)Computational Results:

Coordinates for NBO analysis of 12[K(Et₂O)₃] (from crystallographicstructure): Center Atomic Atom- Num- Num- ic Coordinates (Angstroms) berber Type X Y Z  1 26 0 −1.103223 −0.012183 0.135795  2 19 0 4.315234−0.139004 −0.090250  3 15 0 −1.675324 −1.173876 1.915082  4 15 0−1.065123 −1.018414 −1.816226  5 15 0 −1.284994 2.184405 0.235987  6 8 04.528660 2.280807 −1.344490  7 8 0 5.911867 0.348888 1.940111  8 7 00.612427 −0.071693 0.411598  9 7 0 1.748732 −0.115596 0.595993  10 8 04.713206 −2.836955 −0.525656  11 6 0 −3.683547 −1.101439 −1.107075  12 60 −2.848212 2.944956 2.477923  13 1 0 −2.902445 3.411133 3.314657  14 10 −2.943217 2.002256 2.629817  15 1 0 −3.549324 3.247396 1.896562  16 60 −5.300719 0.467098 1.314827  17 1 0 −5.754021 0.820458 0.583473  18 60 −4.001912 −0.590995 3.509773  19 1 0 −3.561669 −0.948234 4.247172  206 0 −3.352643 −0.565059 2.266253  21 6 0 −5.340093 −2.690118 −1.938757 22 1 0 −6.213047 −3.009821 −1.927115  23 6 0 −3.239989 0.059982−0.210069  24 6 0 −5.939548 0.424855 2.554070  25 1 0 −6.807180 0.7468252.637851  26 6 0 −3.995744 −0.009489 1.140999  27 6 0 −4.738827 1.530724−1.725792  28 1 0 −5.272600 0.789814 −1.900313  29 6 0 −2.8339702.529034 −0.662244  30 6 0 −0.699925 −0.964724 3.508975  31 1 0−1.255246 −1.274126 4.255707  32 6 0 −3.177781 3.758444 −1.223575  33 10 −2.652914 4.506767 −1.050375  34 6 0 −5.064227 2.756173 −2.308949  351 0 −5.796246 2.817735 −2.878437  36 6 0 −1.997936 −3.044284 2.072403 37 1 0 −1.172254 −3.499981 1.807364  38 6 0 −3.630193 1.387339−0.885512  39 6 0 −4.982113 −1.629900 −1.110692  40 1 0 −5.621718−1.261735 −0.545281  41 6 0 −5.290076 −0.092274 3.658747  42 1 0−5.709168 −0.107013 4.489935  43 6 0 −3.084558 −3.496396 1.100751  44 10 −3.186989 −4.449759 1.155643  45 1 0 −2.836970 −3.251424 0.205987  461 0 −3.914011 −3.072883 1.330188  47 6 0 −2.374639 −3.573776 3.467980 48 1 0 −3.208651 −3.185989 3.743407  49 1 0 −1.689325 −3.3361504.096275  50 1 0 −2.461494 −4.529577 3.434330  51 6 0 −3.115644−2.771435 −2.799728  52 1 0 −2.483847 −3.154087 −3.366548  53 6 0−0.750968 −1.148162 −4.740497  54 1 0 −1.580024 −1.620407 −4.840874  551 0 −0.038712 −1.776973 −4.607887  56 1 0 −0.578243 −0.631117 −5.530824 57 6 0 1.502649 −2.065440 −2.315231  58 1 0 1.815103 −1.592208−1.539649  59 1 0 1.549957 −1.489740 −3.081720  60 1 0 2.053171−2.837709 −2.460536  61 6 0 −2.747844 −1.699193 −1.980183  62 6 0−0.325574 0.487045 3.757235  63 1 0 0.179496 0.553496 4.571506  64 1 0−1.124499 1.014869 3.832211  65 1 0 0.203405 0.811036 3.024982  66 6 0−0.022549 −3.492168 −0.929395  67 1 0 0.581790 −4.220856 −1.087339  68 10 −0.916832 −3.829177 −0.851393  69 1 0 0.221701 −3.043661 −0.116590  706 0 −1.487758 3.224594 1.824152  71 1 0 −0.808956 2.902562 2.452414  726 0 0.050193 −2.507521 −2.095661  73 1 0 −0.255203 −2.969370 −2.904599 74 6 0 0.580691 −1.821703 3.461949  75 1 0 1.123702 −1.546872 2.719257 76 1 0 0.343951 −2.746055 3.359005  77 1 0 1.073156 −1.706308 4.278167 78 6 0 −0.838007 −0.212835 −3.526978  79 1 0 0.005710 0.283396−3.493229  80 6 0 5.552067 0.484633 3.317571  81 1 0 5.929145 1.3023763.676051  82 1 0 5.908135 −0.261935 3.825577  83 6 0 7.315816 0.4886071.718843  84 1 0 7.801006 −0.186282 2.219940  85 1 0 7.613666 1.3630182.015963  86 6 0 −1.299822 4.739239 1.722486  87 1 0 −1.914962 5.0963791.078526  88 1 0 −0.400500 4.932966 1.446202  89 1 0 −1.464279 5.1405262.579276  90 6 0 −4.406596 −3.272724 −2.778249  91 1 0 −4.643071−3.990150 −3.321219  92 6 0 −0.039374 3.319208 −0.609736  93 1 0−0.364441 4.240727 −0.526365  94 6 0 5.211785 3.482161 −0.977128  95 1 04.717915 4.251132 −1.301884  96 1 0 6.092941 3.493459 −1.381554  97 6 05.431388 −3.446517 −1.561156  98 1 0 4.909694 −4.171249 −1.941774  99 10 6.257089 −3.817878 −1.214349 100 6 0 1.307915 3.224629 0.107122 101 10 1.676736 2.348031 −0.020042 102 1 0 1.184523 3.386169 1.045478 103 1 01.909864 3.882193 −0.252694 104 6 0 4.068393 0.517784 3.442582 105 1 03.721485 1.265472 2.948807 106 1 0 3.827230 0.606058 4.366826 107 1 03.699349 −0.296154 3.090825 108 6 0 7.581088 0.327433 0.244385 109 1 07.263162 −0.529819 −0.046117 110 1 0 8.524717 0.391569 0.080971 111 1 07.123341 1.019454 −0.239674 112 6 0 −1.943089 0.817273 −3.782952 113 1 0−1.701239 1.365393 −4.533243 114 1 0 −2.051741 1.370207 −3.005635 115 10 −2.768224 0.362999 −3.969832 116 6 0 0.135799 3.014067 −2.094347 117 10 0.816189 3.583836 −2.459410 118 1 0 −0.692505 3.169693 −2.553507 119 10 0.395576 2.095831 −2.203060 120 6 0 5.338946 3.557421 0.519011 121 1 04.464517 3.536775 0.915276 122 1 0 5.781619 4.374392 0.762325 123 1 05.851605 2.809564 0.835438 124 6 0 4.081567 2.325066 −2.694760 125 1 04.843965 2.383314 −3.292513 126 1 0 3.527624 3.108279 −2.831847 127 6 0−4.305306 3.877919 −2.045170 128 1 0 −4.540148 4.701034 −2.407612 129 60 5.733516 −2.440262 −2.609574 130 1 0 4.912970 −2.089325 −2.961565 1311 0 6.234523 −2.854135 −3.317077 132 1 0 6.248687 −1.726357 −2.227458133 6 0 3.297851 1.090761 −2.994716 134 1 0 3.881903 0.327903 −2.972567135 1 0 2.902512 1.166548 −3.865794 136 1 0 2.606298 0.983340 −2.337685137 6 0 3.297906 −3.191439 1.321519 138 1 0 3.844961 −2.661537 1.906929139 1 0 2.836541 −3.860832 1.835131 140 1 0 2.656223 −2.626817 0.883823141 6 0 4.136732 −3.836024 0.327386 142 1 0 4.839780 −4.339808 0.765526143 1 0 3.606577 −4.453089 −0.201928Natural Population and Charge Analysis

Summary of Natural Population Analysis: Atom No Charge Core ValenceRydberg Total Fe  1 −1.46741  17.98394  9.44097 0.04249 27.46741  K  20.64775 17.99326  0.35849 0.00050 18.35225  P  3 1.27701 9.99593 3.655070.07199 13.72299  P  4 1.27393 9.99590 3.65705 0.07311 13.72607  P  51.26960 9.99597 3.66199 0.07244 13.73040  O  6 −0.62206  1.99980 6.601700.02056 8.62206 O  7 −0.62871  1.99981 6.61012 0.01878 8.62871 N  80.13757 1.99930 4.78375 0.07939 6.86243 N  9 −0.38115  1.99962 5.343590.03794 7.38115 O  10 −0.61888  1.99980 6.59853 0.02055 8.61888 C  110.03324 1.99900 3.94356 0.02420 5.96676 C  12 −0.58933  1.99939 4.581940.00799 6.58933 H  13 0.20074 0.00000 0.79796 0.00130 0.79926 H  140.21689 0.00000 0.78114 0.00197 0.78311 H  15 0.21007 0.00000 0.788600.00132 0.78993 C  16 −0.20245  1.99889 4.18565 0.01790 6.20245 H  170.19715 0.00000 0.80047 0.00238 0.80285 C  18 −0.19551  1.99919 4.181520.01480 6.19551 H  19 0.18943 0.00000 0.80850 0.00208 0.81057 C  20−0.36151  1.99893 4.33351 0.02906 6.36151 C  21 −0.19506  1.999334.18256 0.01317 6.19506 H  22 0.19161 0.00000 0.80668 0.00171 0.80839 C 23 −0.24383  1.99889 4.20581 0.03913 6.24383 C  24 −0.19708  1.999334.18432 0.01343 6.19708 H  25 0.19149 0.00000 0.80686 0.00166 0.80851 C 26 0.03137 1.99899 3.94618 0.02346 5.96863 C  27 −0.21664  1.999134.20149 0.01602 6.21664 H  28 0.19763 0.00000 0.80004 0.00233 0.80237 C 29 −0.36260  1.99893 4.33489 0.02879 6.36260 C  30 −0.53199  1.999294.51377 0.01892 6.53199 H  31 0.20509 0.00000 0.79210 0.00281 0.79491 C 32 −0.17988  1.99894 4.16487 0.01608 6.17988 H  33 0.18683 0.000000.81093 0.00223 0.81317 C  34 −0.19064  1.99931 4.17808 0.01325 6.19064H  35 0.19093 0.00000 0.80734 0.00173 0.80907 C  36 −0.52730  1.999264.51037 0.01768 6.52730 H  37 0.20537 0.00000 0.79253 0.00210 0.79463 C 38 0.03888 1.99897 3.93768 0.02447 5.96112 C  39 −0.20266  1.998904.18569 0.01808 6.20266 H  40 0.19621 0.00000 0.80127 0.00252 0.80379 C 41 −0.21185  1.99931 4.19886 0.01368 6.21185 H  42 0.19221 0.000000.80600 0.00180 0.80779 C  43 −0.58951  1.99940 4.58192 0.00819 6.58951H  44 0.20163 0.00000 0.79705 0.00133 0.79837 H  45 0.21463 0.000000.78340 0.00198 0.78537 H  46 0.21065 0.00000 0.78794 0.00140 0.78935 C 47 −0.60288  1.99940 4.59489 0.00858 6.60288 H  48 0.20842 0.000000.78997 0.00161 0.79158 H  49 0.19819 0.00000 0.80052 0.00129 0.80181 H 50 0.20729 0.00000 0.79144 0.00127 0.79271 C  51 −0.18450  1.998944.16936 0.01620 6.18450 H  52 0.18808 0.00000 0.80963 0.00229 0.81192 C 53 −0.60314  1.99941 4.59520 0.00854 6.60314 H  54 0.20821 0.000000.79017 0.00162 0.79179 H  55 0.19684 0.00000 0.80188 0.00127 0.80316 H 56 0.20700 0.00000 0.79174 0.00126 0.79300 C  57 −0.59043  1.999414.58309 0.00792 6.59043 H  58 0.20584 0.00000 0.79087 0.00329 0.79416 H 59 0.20060 0.00000 0.79807 0.00134 0.79940 H  60 0.19874 0.000000.79959 0.00167 0.80126 C  61 −0.36574  1.99894 4.33721 0.02958 6.36574C  62 −0.59173  1.99941 4.58456 0.00776 6.59173 H  63 0.19847 0.000000.80017 0.00135 0.80153 H  64 0.21141 0.00000 0.78727 0.00132 0.78859 H 65 0.21350 0.00000 0.78420 0.00230 0.78650 C  66 −0.58995  1.999404.58279 0.00775 6.58995 H  67 0.19510 0.00000 0.80351 0.00140 0.80490 H 68 0.21447 0.00000 0.78424 0.00129 0.78553 H  69 0.21245 0.000000.78575 0.00180 0.78755 C  70 −0.52833  1.99925 4.51129 0.01779 6.52833H  71 0.20886 0.00000 0.78893 0.00221 0.79114 C  72 −0.53421  1.999294.51541 0.01950 6.53421 H  73 0.20462 0.00000 0.79260 0.00279 0.79538 C 74 −0.59659  1.99940 4.58966 0.00753 6.59659 H  75 0.21373 0.000000.78432 0.00195 0.78627 H  76 0.20192 0.00000 0.79687 0.00121 0.79808 H 77 0.20178 0.00000 0.79689 0.00133 0.79822 C  78 −0.53038  1.999264.51316 0.01796 6.53038 H  79 0.20219 0.00000 0.79557 0.00224 0.79781 C 80 −0.04176  1.99917 4.03024 0.01235 6.04176 H  81 0.17980 0.000000.81822 0.00198 0.82020 H  82 0.18048 0.00000 0.81756 0.00195 0.81952 C 83 −0.04055  1.99918 4.02912 0.01225 6.04055 H  84 0.18398 0.000000.81407 0.00195 0.81602 H  85 0.18176 0.00000 0.81610 0.00214 0.81824 C 86 −0.60429  1.99941 4.59648 0.00840 6.60429 H  87 0.20382 0.000000.79463 0.00155 0.79618 H  88 0.19918 0.00000 0.79960 0.00122 0.80082 H 89 0.20902 0.00000 0.78966 0.00131 0.79098 C  90 −0.21908  1.999334.20583 0.01391 6.21908 H  91 0.19181 0.00000 0.80641 0.00178 0.80819 C 92 −0.53192  1.99929 4.51319 0.01944 6.53192 H  93 0.20485 0.000000.79225 0.00291 0.79515 C  94 −0.04050  1.99917 4.02887 0.01245 6.04050H  95 0.18419 0.00000 0.81373 0.00208 0.81581 H  96 0.17602 0.000000.82199 0.00198 0.82398 C  97 −0.04141  1.99921 4.02880 0.01339 6.04141H  98 0.18298 0.00000 0.81479 0.00223 0.81702 H  99 0.17782 0.000000.82014 0.00204 0.82218 C 100 −0.59222  1.99940 4.58489 0.00793 6.59222H 101 0.21333 0.00000 0.78348 0.00319 0.78667 H 102 0.20373 0.000000.79499 0.00127 0.79627 H 103 0.19554 0.00000 0.80292 0.00154 0.80446 C104 −0.61128  1.99945 4.60495 0.00688 6.61128 H 105 0.21744 0.000000.78076 0.00180 0.78256 H 106 0.22131 0.00000 0.77775 0.00094 0.77869 H107 0.21682 0.00000 0.78151 0.00168 0.78318 C 108 −0.61590  1.999444.60973 0.00673 6.61590 H 109 0.20825 0.00000 0.79001 0.00173 0.79175 H110 0.22330 0.00000 0.77583 0.00088 0.77670 H 111 0.20956 0.000000.78844 0.00200 0.79044 C 112 −0.58856  1.99939 4.58096 0.00821 6.58856H 113 0.20075 0.00000 0.79802 0.00124 0.79925 H 114 0.21848 0.000000.77936 0.00215 0.78152 H 115 0.20985 0.00000 0.78888 0.00127 0.79015 C116 −0.59188  1.99940 4.58480 0.00768 6.59188 H 117 0.19375 0.000000.80484 0.00141 0.80625 H 118 0.21584 0.00000 0.78280 0.00135 0.78416 H119 0.20764 0.00000 0.79076 0.00160 0.79236 C 120 −0.61677  1.999434.61060 0.00674 6.61677 H 121 0.22032 0.00000 0.77814 0.00154 0.77968 H122 0.21640 0.00000 0.78267 0.00092 0.78360 H 123 0.20455 0.000000.79327 0.00218 0.79545 C 124 −0.04114  1.99918 4.02940 0.01256 6.04114H 125 0.17691 0.00000 0.82114 0.00195 0.82309 H 126 0.18667 0.000000.81131 0.00202 0.81333 C 127 −0.21780  1.99933 4.20443 0.01404 6.21780H 128 0.19017 0.00000 0.80811 0.00172 0.80983 C 129 −0.61559  1.999454.60971 0.00643 6.61559 H 130 0.21392 0.00000 0.78443 0.00165 0.78608 H131 0.22119 0.00000 0.77791 0.00090 0.77881 H 132 0.20221 0.000000.79625 0.00154 0.79779 C 133 −0.61285  1.99945 4.60617 0.00723 6.61285H 134 0.19711 0.00000 0.80130 0.00159 0.80289 H 135 0.22545 0.000000.77350 0.00105 0.77455 H 136 0.22507 0.00000 0.77309 0.00183 0.77493 C137 −0.61910  1.99947 4.61242 0.00721 6.61910 H 138 0.20434 0.000000.79398 0.00168 0.79566 H 139 0.22460 0.00000 0.77445 0.00095 0.77540 H140 0.22981 0.00000 0.76837 0.00182 0.77019 C 141 −0.04266  1.999144.03108 0.01244 6.04266 H 142 0.17873 0.00000 0.81929 0.00198 0.82127 H143 0.18493 0.00000 0.81298 0.00209 0.81507Natural Bond Orbital Analysis for Bonds of Interest

(Occupancy) Bond orbital/Coefficients/Hybrids 1. (1.84620) BD (1)Fe1—P3(25.00%) 0.5000*Fe1 s(22.10%)p 2.97(65.54%)d 0.56(12.36%) f 0.00(0.01%)0.0000 0.0000 0.0018 −0.4700 0.0030 0.0000 −0.0006 −0.1440 0.0154 0.0016−0.0079 0.0000 0.0003 −0.4789 0.0132 −0.0005 −0.0004 0.0000 0.00140.6363 −0.0072 0.0047 0.0020 0.0503 0.0003 −0.0007 −0.1079 −0.00870.0016 −0.1041 0.0224 −0.0069 −0.2673 −0.0003 −0.0053 0.1624 −0.00560.0040 −0.0011 0.0011 −0.0075 −0.0013 −0.0018 −0.0005 −0.0016 (75.00%)0.8660* P3 s(45.61%)p 1.19(54.37%)d 0.00(0.02%) 0.0000 0.0009 0.67480.0261 0.0044 −0.0013 0.0004 0.2963 −0.0194 0.0014 0.0002 0.2694 −0.02810.0033 −0.0003 −0.6165 0.0463 0.0044 0.0013 −0.0073 −0.0067 −0.00230.0082 2. (1.84783) BD (1)Fe1—P4 (24.66%) 0.4966*Fe1 s(22.28%)p2.95(65.73%) d 0.54(11.98%) f 0.00(0.01%) 0.0000 0.0000 0.0012 −0.47200.0029 0.0000 −0.0004 0.0875 0.0133 0.0038 −0.0055 0.0000 −0.0013−0.3277 0.0023 −0.0026 −0.0029 0.0000 −0.0005 −0.7360 0.0174 0.0047−0.0025 0.0104 0.0088 −0.0022 −0.0817 0.0103 −0.0062 0.0726 −0.01420.0036 −0.2421 −0.0118 −0.0018 0.2199 −0.0116 0.0064 −0.0014 0.0006−0.0072 −0.0012 0.0020 −0.0015 −0.0022 (75.34%) 0.8680* P4 s(46.11%)p1.17(53.87%)d 0.00(0.02%) 0.0000 0.0008 0.6785 0.0271 0.0038 −0.00170.0003 0.1088 −0.0037 0.0030 0.0002 0.4019 −0.0288 −0.0074 0.0004 0.6016−0.0503 0.0011 0.0019 0.0006 0.0097 −0.0040 0.0062 3. (1.84905) BD(1)Fe1—P5 (24.92%) 0.4992*Fe1 s(22.39%)p 2.91(65.15%)d 0.56(12.45%) f0.00(0.01%) 0.0000 0.0000 0.0018 −0.4732 0.0013 0.0000 −0.0002 −0.00730.0149 0.0020 −0.0063 0.0000 0.0010 0.8020 −0.0182 0.0024 0.0032 0.0000−0.0012 0.0872 −0.0052 −0.0036 −0.0012 0.0023 −0.0101 0.0045 −0.0803−0.0030 −0.0003 0.0345 −0.0077 0.0035 −0.3346 0.0087 −0.0083 0.06520.0176 −0.0019 −0.0011 0.0003 −0.0076 −0.0021 0.0012 −0.0013 −0.0024(75.08%) 0.8665* P5 s(45.74%)p 1.19(54.24%)d 0.00(0.02%) 0.0000 0.00080.6758 0.0266 0.0043 −0.0014 0.0004 0.1620 −0.0073 0.0026 −0.0004−0.7133 0.0572 0.0020 0.0000 0.0627 0.0009 −0.0065 −0.0034 0.0000−0.0020 −0.0112 −0.0053 4. (1.93106) BD (1)Fe1—N8 (20.42%) 0.4519*Fe1s(18.42%)p 2.68(49.34%)d 1.75(32.18%) f 0.00(0.06%) 0.0000 0.0000 0.0014−0.4286 −0.0217 0.0000 −0.0003 0.6896 0.0616 −0.0002 −0.0021 0.00000.0000 −0.0237 −0.0021 0.0002 0.0016 0.0000 −0.0001 0.1161 0.0098 0.0012−0.0010 −0.0345 −0.0021 −0.0003 0.1546 0.0061 0.0013 −0.0024 −0.00130.0002 0.4791 0.0198 0.0040 −0.2581 −0.0108 −0.0023 −0.0051 −0.0130−0.0009 0.0078 −0.0001 0.0185 −0.0022 (79.58%) 0.8921* N8 s(64.45%)p0.55(35.55%)d 0.00(0.00%) 0.0000 0.8016 0.0435 −0.0028 −0.0001 −0.58640.0458 0.0048 0.0212 −0.0016 −0.0002 −0.0950 0.0043 0.0016 −0.00010.0004 0.0000 0.0019 −0.0009 5. (1.76371) BD (1)Fe1—C23 (27.45%)0.5240*Fe1 s(14.70%)p 3.43(50.36%)d 2.38(34.92%) f 0.00(0.03%) 0.00000.0000 0.0024 −0.3833 0.0068 0.0000 −0.0017 −0.6994 −0.0290 0.0077−0.0040 0.0000 0.0001 0.0268 0.0008 −0.0002 −0.0014 0.0000 −0.0002−0.1130 −0.0047 0.0025 0.0005 −0.0362 0.0015 −0.0002 0.1614 −0.00700.0025 −0.0047 −0.0002 −0.0003 0.4992 −0.0195 0.0072 −0.2682 0.0103−0.0036 0.0020 0.0085 −0.0020 −0.0059 0.0005 −0.0120 0.0006 (72.55%)0.8517* C23 s(26.97%)p 2.71(73.03%)d 0.00(0.00%) 0.0007 0.5192 0.00580.0073 0.0001 0.8427 −0.0354 −0.0055 −0.0320 0.0006 0.0004 0.1334−0.0063 −0.0012 0.0004 −0.0009 −0.0001 −0.0024 0.0012 1513. (0.04357)BD*(1)Fe1—P3 (75.00%) 0.8660*Fe1 s(22.10%)p 2.97(65.54%)d 0.56(12.36%) f0.00(0.01%) 0.0000 0.0000 0.0018 −0.4700 0.0030 0.0000 −0.0006 −0.14400.0154 0.0016 −0.0079 0.0000 0.0003 −0.4789 0.0132 −0.0005 −0.00040.0000 0.0014 0.6363 −0.0072 0.0047 0.0020 0.0503 0.0003 −0.0007 −0.1079−0.0087 0.0016 −0.1041 0.0224 −0.0069 −0.2673 −0.0003 −0.0053 0.1624−0.0056 0.0040 −0.0011 0.0011 −0.0075 −0.0013 0.0018 −0.0005 −0.0016(25.00%) −0.5000* P3 s(45.61%)p 1.19(54.37%)d 0.00(0.02%) 0.0000 0.00090.6748 0.0261 0.0044 −0.0013 0.0004 0.2963 −0.0194 0.0014 0.0002 0.2694−0.0281 0.0033 −0.0003 −0.6165 0.0463 0.0044 0.0013 −0.0073 −0.0067−0.0023 0.0082 1514. (0.04400) BD*(1)Fe1—P4 (75.34%) 0.8680*Fe 1s(22.28%)p 2.95(65.73%)d 0.54(11.98%) f 0.00(0.01%) 0.0000 0.0000 0.0012−0.4720 0.0029 0.0000 −0.0004 0.0875 0.0133 0.0038 −0.0055 0.0000−0.0013 −0.3277 0.0023 −0.0026 −0.0029 0.0000 −0.0005 −0.7360 0.01740.0047 −0.0025 0.0104 0.0088 −0.0022 −0.0817 0.0103 −0.0062 0.0726−0.0142 0.0036 −0.2421 −0.0118 −0.0018 0.2199 −0.0116 0.0064 −0.00140.0006 −0.0072 −0.0012 0.0020 −0.0015 −0.0022 (24.66%) −0.4966* P 4s(46.11%)p 1.17(53.87%)d 0.00(0.02%) 0.0000 0.0008 0.6785 0.0271 0.0038−0.0017 0.0003 0.1088 −0.0037 0.0030 0.0002 0.4019 −0.0288 −0.00740.0004 0.6016 −0.0503 0.0011 0.0019 0.0006 0.0097 −0.0040 0.0062 1515.(0.04386) BD*(1)Fe 1 - P 5 (75.08%) 0.8665*Fe1 s(22.39%)p 2.91(65.15%)d0.56(12.45%) f 0.00(0.01%) 0.0000 0.0000 0.0018 −0.4732 0.0013 0.0000−0.0002 −0.0073 0.0149 0.0020 −0.0063 0.0000 0.0010 0.8020 −0.01820.0024 0.0032 0.0000 −0.0012 0.0872 −0.0052 −0.0036 −0.0012 0.0023−0.0101 0.0045 −0.0803 −0.0030 −0.0003 0.0345 −0.0077 0.0035 −0.33460.0087 −0.0083 0.0652 0.0176 −0.0019 −0.0011 0.0003 −0.0076 −0.00210.0012 −0.0013 −0.0024 (24.92%) −0.4992* P5 s(45.74%)p 1.19(54.24%)d0.00(0.02%) 0.0000 0.0008 0.6758 0.0266 0.0043 −0.0014 0.0004 0.1620−0.0073 0.0026 −0.0004 −0.7133 0.0572 0.0020 0.0000 0.0627 0.0009−0.0065 −0.0034 0.0000 −0.0020 −0.0112 −0.0053 1516. (0.08300)BD*(1)Fe1—N8 (79.58%) 0.8921*Fe1 s(18.42%)p 2.68(49.34%)d 1.75(32.18%) f0.00(0.06%) 0.0000 0.0000 0.0014 −0.4286 −0.0217 0.0000 −0.0003 0.68960.0616 −0.0002 −0.0021 0.0000 0.0000 −0.0237 −0.0021 0.0002 0.00160.0000 −0.0001 0.1161 0.0098 0.0012 −0.0010 −0.0345 −0.0021 −0.00030.1546 0.0061 0.0013 −0.0024 −0.0013 0.0002 0.4791 0.0198 0.0040 −0.2581−0.0108 −0.0023 −0.0051 −0.0130 −0.0009 0.0078 −0.0001 0.0185 −0.0022(20.42%) −0.4519* N8 s(64.45%)p 0.55(35.55%)d 0.00(0.00%) 0.0000 0.80160.0435 −0.0028 −0.0001 −0.5864 0.0458 0.0048 0.0212 −0.0016 −0.0002−0.0950 0.0043 0.0016 −0.0001 0.0004 0.0000 0.0019 −0.0009 1517.(0.09062) BD*(1)Fe1—C23 (72.55%) 0.8517*Fe1 s(14.70%)p 3.43(50.36%)d2.38(34.92%) f 0.00(0.03%) 0.0000 0.0000 0.0024 −0.3833 0.0068 0.0000−0.0017 −0.6994 −0.0290 0.0077 −0.0040 0.0000 0.0001 0.0268 0.0008−0.0002 −0.0014 0.0000 −0.0002 −0.1130 −0.0047 0.0025 0.0005 −0.03620.0015 −0.0002 0.1614 −0.0070 0.0025 −0.0047 −0.0002 −0.0003 0.4992−0.0195 0.0072 −0.2682 0.0103 −0.0036 0.0020 0.0085 −0.0020 −0.00590.0005 −0.0120 0.0006 (27.45%) −0.5240* C23 s(26.97%)p 2.71(73.03%)d0.00(0.00%) 0.0007 0.5192 0.0058 0.0073 0.0001 0.8427 −0.0354 −0.0055−0.0320 0.0006 0.0004 0.1334 −0.0063 −0.0012 0.0004 −0.0009 −0.0001−0.0024 0.0012 (0.03482) BD*(1) C11—C23 (48.70%) 0.6978* C11 s(33.58%)p1.98(66.38%)d 0.00(0.03%) −0.0001 −0.5792 −0.0172 −0.0008 0.0009 −0.20990.0082 −0.0022 −0.6201 0.0243 0.0038 −0.4842 0.0106 0.0047 −0.0066−0.0035 −0.0148 0.0080 0.0001 (51.30%) −0.7163* C23 s(24.80%)p3.03(75.13%)d 0.00(0.07%) 0.0002 −0.4979 0.0073 0.0026 0.0001 0.25260.0037 −0.0023 0.6602 0.0177 0.0084 0.5012 0.0072 0.0070 −0.0160 −0.0119−0.0162 0.0080 −0.0005 1537. (0.03133) BD*(1) C11—C39 (49.58%) 0.7041*C11 s(33.03%)p 2.03(66.94%)d 0.00(0.02%) 0.0002 −0.5746 0.0136 0.0025−0.0007 0.7661 0.0163 0.0115 0.2861 −0.0003 −0.0004 −0.0117 −0.00950.0016 −0.0089 −0.0019 0.0018 −0.0092 0.0076 (50.42%) −0.7101* C39s(33.98%)p 1.94(65.94%)d 0.00(0.08%) −0.0002 −0.5829 −0.0049 −0.0014−0.7523 0.0055 0.0184 −0.3044 −0.0110 0.0071 0.0055 −0.0137 −0.0018−0.0173 0.0018 −0.0100 −0.0141 0.0134 1538. (0.31948) BD*(2) C11—C39(53.31%) 0.7301* C11 s(0.01%)p 1.00(99.94%)d 0.00(0.05%) 0.0000 −0.0091−0.0008 0.0005 0.0025 −0.2612 0.0062 −0.0044 0.6523 −0.0040 0.0005−0.7110 0.0039 −0.0054 −0.0060 0.0132 0.0050 0.0137 0.0093 (46.69%)−0.6833* C39 s(0.00%)p 1.00(99.97%)d 0.00(0.02%) −0.0001 −0.0048 0.00230.0001 −0.2638 0.0066 0.0017 0.6482 −0.0144 −0.0055 −0.7138 0.01290.0081 0.0090 −0.0090 −0.0039 −0.0043 0.0067 1558. (0.03658) BD*(1)C23—C26 (51.30%) 0.7162* C23 s(23.91%)p 3.18(76.01%)d 0.00(0.07%)−0.0003 0.4890 −0.0055 −0.0027 −0.0003 −0.4236 −0.0066 0.0002 −0.04700.0005 −0.0019 0.7603 0.0164 0.0102 0.0010 −0.0242 −0.0001 0.0069 0.0093(48.70%) −0.6978* C26 s(32.91%)p 2.04(67.05%)d 0.00(0.03%) 0.0002 0.57340.0183 0.0001 −0.0007 0.3768 −0.0128 0.0010 0.0404 0.0037 −0.0022−0.7253 0.0264 0.0054 −0.0006 −0.0125 −0.0013 0.0037 0.0127 1559.(0.03587) BD*(1) C23—C38 (51.26%) 0.7160* C23 s(24.27%)p 3.12(75.66%)d0.00(0.07%) −0.0003 0.4926 −0.0058 −0.0027 0.0002 −0.2136 −0.0013 0.00270.7478 0.0151 0.0099 −0.3889 −0.0133 −0.0023 −0.0168 0.0086 −0.0151−0.0112 −0.0028 (48.74%) −0.6981* C38 s(33.14%)p 2.02(66.83%)d0.00(0.03%) 0.0002 0.5754 0.0159 0.0008 −0.0006 0.1860 −0.0064 0.0016−0.7162 0.0203 0.0055 0.3463 −0.0171 −0.0001 −0.0052 0.0044 −0.0119−0.0114 −0.0037Second-Order Perturbation Analysis: Donor-Acceptor InteractionsInvolving the Fe—C a Bond:

Second Order Perturbation Theory Analysis of Fock Matrix in NBO BasisThreshold for printing: 0.50 kcal/mol E(2) E(j)- kcal/ E(i) F(i, j)Donor NBO (i) Acceptor NBO (j) mol a.u. a.u. 5. BD (1)Fe1—C23 /263.RY*(1)Fe1 0.57 0.86 0.021 5. BD (1)Fe1—C23 /266. RY*(4)Fe1 0.87 0.930.027 5. BD (1)Fe1—C23 /300. RY*(2) P3 0.63 1.49 0.029 5. BD (1)Fe1—C23/303. RY*(5) P3 1.04 1.17 0.033 5. BD (1)Fe1—C23 /305. RY*(7) P3 1.111.45 0.038 5. BD (1)Fe1—C23 /315. RY*(3) P4 0.53 1.36 0.025 5. BD(1)Fe1—C23 /317. RY*(5) P4 0.90 1.14 0.030 5. BD (1)Fe1—C23 /319. RY*(7)P4 1.72 1.38 0.046 5. BD (1)Fe1—C23 /328. RY*(2) P5 0.57 1.51 0.028 5.BD (1)Fe1—C23 /329. RY*(3) P5 0.63 1.36 0.028 5. BD (1)Fe1—C23 /331.RY*(5) P5 0.67 1.18 0.027 5. BD (1)Fe1—C23 /333. RY*(7) P5 1.90 1.440.050 5. BD (1)Fe1—C23 /371. RY*(3) N8 3.86 1.19 0.064 5. BD (1)Fe1—C23/378. RY*(10) N8 1.14 1.87 0.044 5. BD (1)Fe1—C23 /412. RY*(2) C11 2.101.13 0.046 5. BD (1)Fe1—C23 /432. RY*(8) C12 3.50 0.47 0.038 5. BD(1)Fe1—C23 /434. RY*(10) C12 1.29 1.63 0.044 5. BD (1)Fe1—C23 /453.RY*(5) H15 0.80 2.95 0.046 5. BD (1)Fe1—C23 /496. RY*(6) C20 13.77  0.190.048 5. BD (1)Fe1—C23 /512. RY*(8) C21 3.90 0.42 0.039 5. BD (1)Fe1—C23/513. RY*(9) C21 3.23 0.66 0.044 5. BD (1)Fe1—C23 /524. RY*(1) C23 3.811.47 0.071 5. BD (1)Fe1—C23 /531. RY*(8) C23 2.41 3.06 0.082 5. BD(1)Fe1—C23 /545. RY*(8) C24 0.62 1.10 0.025 5. BD (1)Fe1—C23 /548.RY*(11) C24 1.47 0.68 0.030 5. BD (1)Fe1—C23 /569. RY*(13) C26 0.56 1.460.027 5. BD (1)Fe1—C23 /584. RY*(14) C27 4.48 0.63 0.051 5. BD(1)Fe1—C23 /588. RY*(4) H28 2.49 1.14 0.051 5. BD (1)Fe1—C23 /615.RY*(12) C30 5.84 0.42 0.047 5. BD (1)Fe1—C23 /631. RY*(9) C32 1.63 1.140.041 5. BD (1)Fe1—C23 /652. RY*(12) C34 5.10 0.35 0.040 5. BD(1)Fe1—C23 /676. RY*(3) H37 0.51 1.83 0.029 5. BD (1)Fe1—C23 /677.RY*(4) H37 1.46 1.03 0.037 5. BD (1)Fe1—C23 /700. RY*(8) C39 0.63 2.530.038 5. BD (1)Fe1—C23 /735. RY*(6) C43 2.67 0.62 0.038 5. BD (1)Fe1—C23/736. RY*(7) C43 0.80 0.84 0.025 5. BD (1)Fe1—C23 /739. RY*(10) C43 0.591.10 0.024 5. BD (1)Fe1—C23 /743. RY*(14) C43 3.25 1.35 0.063 5. BD(1)Fe1—C23 /754. RY*(1) H46 5.82 0.38 0.045 5. BD (1)Fe1—C23 /757.RY*(4) H46 0.80 1.10 0.028 5. BD (1)Fe1—C23 /764. RY*(6) C47 1.28 1.170.037 5. BD (1)Fe1—C23 /776. RY*(4) H48 0.65 0.54 0.018 5. BD (1)Fe1—C23/785. RY*(3) H50 4.77 0.96 0.064 5. BD (1)Fe1—C23 /794. RY*(7) C51 0.551.71 0.029 5. BD (1)Fe1—C23 /795. RY*(8) C51 3.39 0.73 0.047 5. BD(1)Fe1—C23 /804. RY*(4) H52 0.60 1.90 0.032 5. BD (1)Fe1—C23 /808.RY*(3) C53 2.92 0.71 0.043 5. BD (1)Fe1—C23 /822. RY*(3) H54 0.82 2.330.042 5. BD (1)Fe1—C23 /827. RY*(3) H55 0.52 2.36 0.033 5. BD (1)Fe1—C23/833. RY*(4) H56 0.55 0.49 0.016 5. BD (1)Fe1—C23 /846. RY*(12) C57 0.672.21 0.037 5. BD (1)Fe1—C23 /848. RY*(14) C57 7.14 0.79 0.071 5. BD(1)Fe1—C23 /850. RY*(2) H58 1.26 0.70 0.028 5. BD (1)Fe1—C23 /854.RY*(1) H59 1.34 0.76 0.030 5. BD (1)Fe1—C23 /857. RY*(4) H59 3.54 0.540.041 5. BD (1)Fe1—C23 /859. RY*(1) H60 0.65 0.91 0.023 5. BD (1)Fe1—C23/861. RY*(3) H60 1.22 1.97 0.047 5. BD (1)Fe1—C23 /864. RY*(1) C61 0.881.33 0.032 5. BD (1)Fe1—C23 /890. RY*(13) C62 0.72 1.59 0.032 5. BD(1)Fe1—C23 /891. RY*(14) C62 0.55 1.73 0.029 5. BD (1)Fe1—C23 /901.RY*(5) H64 3.15 1.83 0.072 5. BD (1)Fe1—C23 /910. RY*(4) C66 2.18 0.960.044 5. BD (1)Fe1—C23 /918. RY*(12) C66 2.46 1.24 0.053 5. BD(1)Fe1—C23 /928. RY*(3) H68 13.25  0.43 0.071 5. BD (1)Fe1—C23 /930.RY*(5) H68 0.91 2.71 0.047 5. BD (1)Fe1—C23 /931. RY*(1) H69 1.12 0.980.032 5. BD (1)Fe1—C23 /932. RY*(2) H69 10.78  0.55 0.074 5. BD(1)Fe1—C23 /935. RY*(5) H69 1.12 2.62 0.051 5. BD (1)Fe1—C23 /961.RY*(7) C72 0.76 0.74 0.023 5. BD (1)Fe1—C23 /966. RY*(12) C72 19.18 0.24 0.065 5. BD (1)Fe1—C23 /978. RY*(5) C74 1.79 0.86 0.037 5. BD(1)Fe1—C23 /979. RY*(6) C74 5.12 0.75 0.059 5. BD (1)Fe1—C23 /980.RY*(7) C74 1.45 0.87 0.034 5. BD (1)Fe1—C23 /982. RY*(9) C74 4.59 0.920.062 5. BD (1)Fe1—C23 /985. RY*(12) C74 2.97 1.37 0.061 5. BD(1)Fe1—C23 /992. RY*(5) H75 1.19 2.22 0.049 5. BD (1)Fe1—C23 /***.RY*(6) C78 3.08 1.03 0.054 5. BD (1)Fe1—C23 /***. RY*(12) C78 0.96 1.330.034 5. BD (1)Fe1—C23 /***. RY*(2) C86 1.36 0.77 0.031 5. BD (1)Fe1—C23/***. RY*(5) C86 1.23 0.97 0.033 5. BD (1)Fe1—C23 /***. RY*(7) C86 0.811.27 0.030 5. BD (1)Fe1—C23 /***. RY*(8) C86 0.51 0.98 0.021 5. BD(1)Fe1—C23 /***. RY*(9) C86 1.16 1.40 0.038 5. BD (1)Fe1—C23 /***.RY*(1) H89 0.54 0.92 0.021 5. BD (1)Fe1—C23 /***. RY*(2) H89 9.27 0.670.075 5. BD (1)Fe1—C23 /***. RY*(4) C90 10.95  0.31 0.055 5. BD(1)Fe1—C23 /***. RY*(9) C90 0.77 0.85 0.024 5. BD (1)Fe1—C23 /***.RY*(11) C90 0.73 0.75 0.022 5. BD (1)Fe1—C23 /***. RY*(13) C92 1.62 0.850.035 5. BD (1)Fe1—C23 /***. RY*(4) H102 1.02 0.67 0.025 5. BD(1)Fe1—C23 /***. RY*(5) H103 0.88 2.79 0.047 5. BD (1)Fe1—C23 /***.RY*(4) C112 2.27 0.90 0.043 5. BD (1)Fe1—C23 /***. RY*(6) C112 2.37 0.650.037 5. BD (1)Fe1—C23 /***. RY*(7) C112 2.49 0.74 0.041 5. BD(1)Fe1—C23 /***. RY*(8) C112 0.58 1.91 0.032 5. BD (1)Fe1—C23 /***.RY*(4) H113 2.34 0.39 0.029 5. BD (1)Fe1—C23 /***. RY*(6) C116 4.87 0.490.047 5. BD (1)Fe1—C23 /***. RY*(7) C116 10.17  0.31 0.053 5. BD(1)Fe1—C23 /***. RY*(4) H117 2.79 0.41 0.032 5. BD (1)Fe1—C23 /***.RY*(1) H119 0.79 0.89 0.025 5. BD (1)Fe1—C23 /***. RY*(4) H119 1.65 0.920.037 5. BD (1)Fe1—C23 /***. RY*(4) C127 4.63 0.83 0.059 5. BD(1)Fe1—C23 /***. RY*(3) H128 3.11 1.36 0.062 5. BD (1)Fe1—C23 /***.BD*(1)Fe1—P3 7.31 0.66 0.065 5. BD (1)Fe1—C23 /***. BD*(1)Fe1—P4 6.040.64 0.058 5. BD (1)Fe1—C23 /***. BD*(1)Fe1—P5 7.73 0.66 0.067 5. BD(1)Fe1—C23 /***. BD*(1)Fe1—N8 8.40 0.63 0.068 5. BD (1)Fe1—C23 /***.BD*(1)Fe1—C23 4.92 0.55 0.048 5. BD (1)Fe1—C23 /***. BD*(1) P3—C20 2.430.53 0.034 5. BD (1)Fe1—C23 /***. BD*(1) P3—C30 3.29 0.49 0.037 5. BD(1)Fe1—C23 /***. BD*(1) P3—C36 0.77 0.47 0.018 5. BD (1)Fe1—C23 /***.BD*(1) P4—C61 2.52 0.53 0.034 5. BD (1)Fe1—C23 /***. BD*(1) P4—C72 3.780.48 0.040 5. BD (1)Fe1—C23 /***. BD*(1) P4—C78 0.93 0.47 0.019 5. BD(1)Fe1—C23 /***. BD*(1) P5—C29 2.49 0.53 0.034 5. BD (1)Fe1—C23 /***.BD*(1) P5—C70 0.95 0.47 0.020 5. BD (1)Fe1—C23 /***. BD*(1) P5—C92 3.170.48 0.036 5. BD (1)Fe1—C23 /***. BD*(1) C11—C39 6.70 0.86 0.071 5. BD(1)Fe1—C23 /***. BD*(2) C11—C39 1.46 0.31 0.019 5. BD (1)Fe1—C23 /***.BD*(1) C20—C26 5.95 0.43 0.048 5. BD (1)Fe1—C23 /***. BD*(1) C23—C261.21 0.68 0.027 5. BD (1)Fe1—C23 /***. BD*(1) C23—C38 4.61 0.55 0.048 5.BD (1)Fe1—C23 /***. BD*(1) C27—C38 5.99 0.86 0.068 5. BD (1)Fe1—C23/***. BD*(2) C27—C38 2.45 0.32 0.026 5. BD (1)Fe1—C23 /***. BD*(1)C66—H67 9.18 0.27 0.047 5. BD (1)Fe1—C23 /***. BD*(1) C66—C72 2.11 0.490.031 5. BD (1)Fe1—C23 /***. BD*(1) C74—H75 2.73 0.68 0.041 5. BD(1)Fe1—C23 /***. BD*(1) C112—H113 0.68 0.82 0.022 5. BD (1)Fe1—C23 /***.BD*(1) C116—H117 3.29 0.51 0.039

ADDITIONAL REFERENCES

-   ¹ Kliegl, A. Ber. Deutsch. Chem. Ges. 1908, 40, 4937-42.-   ² Watt, G. W.; Chrisp, J. D. Anal. Chem. 1952, 24, 2006.

EXAMPLE 3 Conversion of Fe—NH₂ to Fe—N₂ with Release of NH₃

Tris(phosphine)borane ligated Fe(I) centers featuring N₂H₄, NH₃, NH₂,and OH ligands are described herein. The conversion of Fe—NH₂ to Fe—NH₃⁺ by addition of acid, and subsequent reductive release of NH₃ togenerate Fe—N₂, is demonstrated. This sequence models the final steps ofproposed Fe-mediated nitrogen fixation pathways. The five-coordinatetrigonal bipyramidal complexes described are unusual in that they adoptS=3/2 ground states and are prepared from a four-coordinate, S=3/2trigonal pyramidal precursor.

Due the structural and mechanistic complexity of biological nitrogenfixation¹ a variety of mechanisms have been proposed that invoke eitherMo or Fe as the likely active site for N₂ binding and reduction. Fe—NH₂is an intermediate common to both limiting mechanisms (i.e., distal vs.alternating) being considered for Fe-mediated N₂ fixation scenarios atthe FeMo-cofactor.^(2,3) Such a species may form either via reductiveprotonation of the nitride intermediate of a distal scheme (i.e.,Fe(N)→Fe(NH)→Fe(NH₂)→Fe(NH₃)), or by reductive protonation of ahydrazine intermediate of an alternating scheme (i.e.,Fe(NH₂—NH₂)→Fe(NH₂)+NH₃). In the latter context, detection of an EPRactive Fe—NH₂ or possibly Fe—NH₃ common intermediate has been proposedunder reducing conditions at the FeMo-cofactor from substrates includingN₂, N₂H₄, and MeN═NH.^(3a)

One key to realizing a catalytic cycle in either limiting scenarioconcerns the regeneration of Fe—N₂ from Fe—NH₂ with concurrent releaseof NH₃.⁴ While there have been recent synthetic reports demonstratingNH₃ generation from Fe(N) nitride model complexes, these studies havenot provided information about the plausible downstream Fe(NH_(x)) (X=1,2, 3) intermediates en route to NH₃ release, nor have these systemsillustrated the feasibility of regeneration of Fe—N₂.⁵ Herein wedescribe a terminal, S=3/2 Fe—NH₂ complex for which the stepwiseconversion to Fe—NH₃, and then to Fe—N₂ along with concomitant releaseof NH₃, is demonstrated (eqns 1 and 2).Fe—NH₂+H⁺→Fe—NH₃ ⁺  (1)Fe—NH₃ ⁺ +e ⁻+N₂→Fe—N₂+NH₃  (2)

Addition of methyllithium to (TPB)FeBr⁶ affords the corresponding methylcomplex (TPB)FeMe (1) in high yield (Scheme 1). Protonation of 1 byHBAr^(F) ₄.2Et₂O (BAr^(F) ₄ ⁻═B(3,5-C₆H₃(CF₃)₂)₄ ⁻) in a cold etherealsolution releases methane to yield [(TPB)Fe][BAr^(F) ₄] (2) which servesas a useful synthon with a vacant coordination site.

XRD data were obtained for 1 and 2 (FIG. 52). The geometry of 1 ispseudo trigonal bipyramidal about Fe with an Fe—C bond length of2.083(10) Å and an Fe—B bond length of 2.522 (2) Å. In the solid state 2possesses a four-coordinate distorted trigonal pyramidal geometry withno close contacts in the apical site trans to boron, making this complexcoordinatively unsaturated. Additionally, there is one wide P—Fe—P angleof 136°. The origin of this large angle is not clear, but suggestsincreased backbonding from a relatively electron rich Fe center into thephosphine ligands that would arise from this distortion.

The Fe—B distance in 2 (2.217(2) Å) is markedly shorter than that in(TPB)FeBr (2.459(5) Å), which is noteworthy because one might expect theloss of an anionic σ-donor ligand to reduce the Lewis basicity of themetal and thus weaken the Fe—B bond. For example, the Au—B distance in(TPB)AuCl (2.318 Å) lengthens upon chloride abstraction to 2.448 Å in[(TPB)Au]⁺.⁷ To explain this difference we note that the boron center infour-coordinate 2 is less pyramidalized (Σ(C—B—C)=347.3°) than that infive-coordinate (TPB)FeBr (Σ(C—B—C)=341.2°), pointing to a weakinteraction despite the short distance. These observations suggest thatthe geometry of 2 might be best understood as derived from a planarthree-coordinate Fe(I) center distorted towards a T-shaped geometry,⁸the unusually short Fe—B distance being due largely to the constraintsimposed by the ligand cage structure. This interpretation is consistentwith a computational model study: the DFT (B3LYP/6-31 G(d)) optimizedgeometry of the hypothetical complex [(Me₂PhP)₃Fe]⁺ exhibits a planargeometry with P—Fe—P angles of 134.8°, 113.1°, and 111.7°, very close tothose measured for [(TPB)Fe]⁺ (137.5°, 113.2°, 109.1°).

When considering the bonding of the (Fe—B)⁷ subunit of 2 to estimate thebest oxidation state and valence assignment, two limiting scenariospresent themselves: Fe(III)/B(I) and Fe(I)/B(III). The structural dataand computations for 2 are suggestive of a weak Fe—B interaction andindicate that this species is better understood to be Fe(I)/B(III)rather than Fe(III)/B(I). Calculations indicate that a small amount ofspin density resides on the B-atom of 2 (Sl) and suggest somecontribution from an Fe(II)/B(II) resonance form is also relevant. Therest of the complexes 3-6 presented herein possess significantly longer,and presumably weaker, Fe—B interactions (vide infra) and are hence alsobetter classified as Fe(I) species. Additional spectroscopic studies(e.g., XAS and Mössbauer) will help to better map the Fe—B bondinginteraction across the variable Fe—B distances and also the spin statesof the complexes. These studies would thereby help to determine thevalue and limitation of classically derived oxidation/valenceassignments for boratranes of these types.⁹

TABLE 24 Selected Metrics for Complexes 1-6 Avg. Σ Σ Fe—X (Å) Fe—B (Å)Fe—P (Å) P—Fe—P C—B—C 1  2.083 (10) 2.523 (2) 2.40 339° 341° 2 — 2.217(2) 2.38 359° 347° 3 2.205 (2) 2.392 (2) 2.44 350° 339° 4 2.280 (3)2.433 (3) 2.44 349° 341° 5 1.918 (3) 2.449 (4) 2.39 343° 339° 6 1.8916(7)  2.4438 (9)  2.39 348° 337°

Solutions of 2 are orange in Et₂O and pale yellow-green in THF.Titration of THF into an ethereal solution of 2 results in a distinctchange in the UV-vis spectrum consistent with weak THF binding. Additionof an excess of N₂H₄ to an ethereal solution of 2 results in a slightlightening of the orange color of the solution to afford[(TPB)Fe(N₂H₄)][BAr^(F) ₄] (3) in 89% yield. Complex 3 shows aparamagnetically shifted ¹H NMR spectrum indicative of an S=3/2 Fecenter that is corroborated by a room temperature solution magneticmoment, μ_(eff), of 3.5 μ_(B). Crystals of 3 were obtained and XRDanalysis (FIG. 53A) indicates a distorted trigonal bipyramidal geometry.The Fe—N distance of 2.205(2) Å is unusually long (2.14 Å is the averagequaternary N—Fe distance in the Cambridge Structural Database)reflecting its unusual quartet spin state.

Complex 3 is stable to vacuum, but solutions decompose cleanly at roomtemperature over hours to form the cationic ammonia complex[(TPB)Fe(NH₃)][BAr^(F) ₄], 4, which was assigned by comparison of its ¹HNMR spectrum with an independently prepared sample formed by theaddition of NH₃ to the cation 2. Analysis of additional degradationproducts shows only NH₃ and trace H₂ (Sl). The assignment of 4 as anammonia adduct was confirmed by XRD analysis (FIG. 53B). Like 3, complex4 shows a long Fe—N distance of 2.280(3) Å in the solid state. Thecomplexes 2, 3 and 4 are unusual by virtue of their S=3/2 spin statesand underscore the utility of local 3-fold symmetry with respect tostabilizing high spin states at iron, even in the presence ofstrong-field phosphine ligands.

Addition of NaNH₂ to the cation 2 affords the terminal amide,(TPB)Fe—NH₂ (5) in ca. 85% non-isolated yield by ¹H NMR integration. TheXRD structure of 5 (FIG. 53C) shows an overall geometry similar to thatobserved in 1, 3, and 4. Of interest is the short Fe—N distance of1.918(3) Å by comparison to 4 (2.280(3) Å). The amide hydrogens werelocated in the difference map and indicate a nearly planar geometryabout N (with the sum of the angles around N being 355°).

While the XRD data set of 5 is of high quality, we were concerned aboutthe difficulty in distinguishing an Fe—NH₂ group from a potentiallydisordered Fe—OH moiety. We therefore independently characterized thehydroxo complex, (TPB)Fe—OH (6) (Scheme 2), which possesses a geometrysimilar to that observed in 5 with an Fe—B distance of 2.4438(9) Å andan Fe—O distance of 1.8916(7) Å. Despite the structural similaritybetween 5 and 6, different spectral signatures in both their ¹H NMR andEPR (FIG. 54) spectra allow for facile distinction between them. Like 2,3, and 4, both 5 and 6 are S=3/2.

Low-temperature EPR data (FIG. 54) have been obtained on complexes 1-6.All complexes show features shifted to large g-values consistent withquartet Fe species.¹⁰ This assignment is verified by the solutionmagnetic moments obtained for these complexes. Variable temperaturesolid-state SQUID magnetic data for complexes 2-5 (Sl) also establishquartet spin state assignments and display no evidence forspin-crossover phenomena. These data show a drop in magnetic moment inthe range 50-70 K for all compounds studied. We propose that this effectis due to a large zero-field splitting in these species, which isconsistent with Fe centers in related geometries.¹¹ Simulations withzero-field splitting of 10-20 cm⁻¹ provide reasonable fits to the data.

Parent amide complexes of first row transition metals are rare.¹²Noteworthy precedent for related terminal M-NH₂ species includes twosquare planar nickel complexes^(12a,d) and one octahedral anddiamagnetic iron complex, (dmpe)₂Fe(H)NH₂.^(12e) In addition to theirdifferent coordination numbers, geometries, and spin-states,(dmpe)₂Fe(H)(NH₂) and 5 show a distinct difference at the Fe—NH₂subunit. Six-coordinate (dmpe)₂Fe(H)(NH₂) is an 18-electron specieswithout π-donation from the amide ligand, which is pyramidalized as aresult. By contrast, five-coordinate 5 accommodates π-bonding from theamide. This is borne out in its much shorter Fe—N distance (1.918(3) Åfor 5 vs 2.068 Å for (dmpe)₂Fe(H)(NH₂)), and also its comparativeplanarity (the sum of the angles around N is 355° for 5 vs 325° for(dmpe)₂Fe(H)(NH₂)).

The terminal amide 5 was then analyzed to determine its suitability as aprecursor to the N₂ complex (TPB)Fe(N₂) via release of NH₃ and toexamine the reduction/protonation vs protonation/reduction sequences asa means of effecting overall H-atom transfer to the Fe—NH₂ unit.Attempts to carry out the one-electron reduction of 5 were notinformative. For example, electrochemical studies of 5 in THF fails toshow any reversible reduction waves, but the addition of harshreductants (e.g., tBuLi) to 5 shows small amounts of (TPB)Fe(N₂) in theproduct profile. A more tractable conversion sequence utilizesprotonation followed by chemical reduction. Thus, the addition ofHBAr^(F) ₄.2Et₂O to 5 at low temperature (−35° C.) rapidly generates thecationic ammonia adduct 4. The conversion is quantitative as determinedby ¹H NMR spectroscopy, and 4 can be isolated in ca. 90% yield from thesolution. Subsequent exposure of 4 to one equiv of KC₈ under anatmosphere of N₂ releases NH₃ and generates the (TPB)FeN₂ complex insimilarly high yield.

In summary, an unusual series of S=3/2 iron complexes featuringterminally bonded N₂H₄, NH₃, NH₂, and OH functionalities has beenthoroughly characterized. These complexes are supported by atris(phosphine)borane ligand and are best described as Fe(I) speciesthat feature weak Fe—B bonding, though other resonance contributions tothe bonding scheme warrant additional consideration. The Fe—NH₂ speciesfaithfully models the reductive replacement of the terminal NH₂ group byN₂ with concomitant release of NH₃, lending credence to such a pathwayas mechanistically viable in Fe-mediated N₂ reduction schemes. Becausespectroscopic detection of a common Fe—NH₂ or Fe—NH₃ intermediate underreductive turnover of the FeMo-cofactor has been recently proposed,³ EPRactive model complexes of the types described here should prove usefulfor comparative purposes.

REFERENCES

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Unless otherwise noted, all compounds were prepared by literatureprocedures or purchased from commercial sources. All manipulations werecarried out under a dinitrogen atmosphere by utilizing standard gloveboxor schlenk techniques. Solvents were dried and de-oxygenated by an argonsparge followed by passage through an activated alumina column purchasedfrom S.G. Waters Company. All non-halogenated solvents were tested witha standard sodium-benzophenone ketyl solution to ensure the absence ofoxygen and water.

NMR

NMR measurements were obtained on Varian 300, 400, or 500 MHzspectrometers. Deuterated solvents for these measurements were obtainedfrom Cambridge Isotope Laboratories and were dried and degassed prior touse. All ¹H spectra were referenced to residual solvent peaks and all³¹P spectra were referenced to an external H₃PO₄ standard.

EPR

EPR X-band spectra were obtained on a Bruker EMX spectrometer with theaid of Bruker Win-EPR software suite version 3.0. The spectrometer wasequipped with a rectangular cavity which operated in the TE₁₀₂ mode.Temperature control was achieved with the use of an Oxfordcontinuous-flow helium cryostat (temperature range 3.6-300 K). Allspectra were recorded at 9.37 GHz with a microwave power of 20 mW, amodulation amplitude of 4 G, and a modulation frequency of 100 kHz.

X-Ray Crystallography

Data was obtained at low temperatures on a Siemens or Bruker Platformthree-circle diffractometer coupled to a Bruker-AXS Smart Apex CCDdetector with graphite-monochromated Mo Kα radiation (λ=0.71073),performing φ-and ω-scans. Data for complex 4 was collected on withsynchrotron radiation at the Stanford Synchrotron Radiation Laboratory(SSRL) beam line 12-2 at 17 keV using a single phi axis and recorded ona Dectris Pilatus 6M. The images were processed using XDS¹ and furtherworkup of the data was analogous to the other datasets. All structureswere solved by standard direct or Patterson methods and refined againstF² using the SHELX program package.^(2,3,4) All atoms, with theexception of hydrogens, have been anisotropically refined. The hydrogenatoms bonded to atoms of interest, namely N or O, have been located inthe difference map and refined semi-freely. All other hydrogen atomswere included via a standard riding model.

In the structure of complex 1 a minor component of (TPB)FeCl was foundin the difference map and modeled as disorder. Additional disorder ofthe BAr^(F) ₄ counterion was found in complex 4. This disorder wasmodeled as a rotational disorder of the CF₃ groups on one of the phenylrings, but some of the resulting F ellipsoids still displaysignificantly prolate shapes. We feel that the shape of these ellipsoidsaccurately describes the actual electron density due to the rotationaldisorder.

Magnetic Measurements.

Data was obtained using a Quantum Designs SQUID magnetometer runningMPMSR2 software (Magnetic Property Measurement System Revision 2) at afield strength of 50000 G. Complexes were massed and then suspended ineicosane wax. Samples were then inserted into the magnetometer inplastic straws sealed under nitrogen with gelatin capsules. Loadedsamples were centered within the magnetometer using the DC centeringscan at 35 K. Data were acquired at 20-30 K (one data point every 2 K),and 30-300 K (one data point every 10 K). The magnetic susceptibilitywas adjusted for diamagnetic contributions using the constitutivecorrections of Pascal's constants as well as a diamagnetic correctionfor the eicosane and capsule. Data workup, including simulations, wasperformed in the JulX software package.⁵ Complex 5 displayed a lowerthan expected magnetic moment. NMR analysis of the sample indicated thepresence of ˜15% 12-crown-4, present as a result of the protocol forgeneration of the complex, where it is used to aid for removal ofNaBAr^(F) ₄. Accounting for this impurity leads to a magnetic momentconsistent with the other samples.

Computational Methods

Geometry optimizations were performed using the Gaussian03 package.⁶ TheB3LYP exchange-correlation functional was employed with a 6-31G(d) basisset. The GDIIS algorithm was used. A full frequency calculation wasperformed on each structure to establish true minima. A model for theinitial geometry of complex 2 used the crystallographically determinedcoordinates as a starting point for subsequent minimization. Atoms werethen stripped away from this structure to reveal a Fe(PMe₂Ph)₃ ⁺ as thestarting point to determine the theoretical structure of Fe(PMe₂Ph)₃+ byanother minimization. Structural models and orbital/spin densitypictures were generated from Gaussview 03.

Synthesis of (TPB)FeMe, 1

(TPB)FeBr (0.400 g, 0.55 mmol) was dissolved in 15 mL of ether andcooled to −35° C. To this stirred solution was added a 1.6 M solution ofMeLi in ether (0.620 mL, 0.99 mmol). After addition the solution wasallowed to warm to room temperature and was stirred for an additionalhour over which time the solution changed in color from a dark brown toa deep orange red. After this time, volatiles were removed and theremaining solids were extracted with 3 mL of benzene three times.Lyophilization of benzene resulted in a dark orange powder which waswashed with 5 mL of cold pentane to yield (TPB)FeMe (0.352 g, 97%).X-ray quality crystals were grown from slow evaporation of aconcentrated pentane solution of 1. ¹H NMR (C₆D₆, δ): 74.48 (br s),33.25 (s), 22.52 (s), 9.31 (br s), 5.73 (s), 2.65 (s), −2.33 (br s),−2.80 (s), −7.49 (br s), −16.33 (s). UV-Vis (THF) λ_(max), nm (ε, M⁻¹cm⁻¹): 840 (120). Anal. Calc. for C₃₇H₅₇BFeP₃: C, 67.19; H, 8.69. Found:C, 67.26; H, 8.59. Solution magnetic moment (C₆D₆): 3.9 μ_(B).

Synthesis of [(TPB)Fe][BAr^(F) ₄] 2

A dark orange solution of 1 (0.037 g, 0.06 mmol) in 5 mL of Et₂O wascooled to −35° C. Once cooled, the solution was stirred while asimilarly cooled solution of HBAr^(F) ₄.2 Et₂O⁷ in 5 mL of Et₂O wasadded dropwise over 5 min. After the addition, the solution was stirredat room temperature for an additional hour before being concentrateddown to 1 mL. This solution was layered with 1 mL of pentane and cooledto −35° C. for 2 days upon which time dark orange crystals of[(TPB)Fe][BAr^(F) ₄] had formed (0.082 g, 97%). ¹H NMR (C₆D₆/THF-d₈, δ):32.15 (br s), 25.78 (s), 23.99 (br s), 8.93 (br s), 8.27 (s, BAr^(F) ₄),4.55 (br s), 1.84 (br s), −1.24 (br s), −28.05 (s). UV-Vis (Et₂O)λ_(max), nm (ε, M⁻¹ cm⁻¹): 475 (1700), 765 (800). Anal. Calc. forC₆₈H₆₆B₂F₂₄FeP₃: C, 54.10; H, 4.41. Found: C, 53.93; H, 4.53.

Synthesis of [(TPB)Fe(N₂H₄)][BAr^(F) ₄], 3

2 (0.356 g, 0.24 mmol) was dissolved in 10 mL in Et₂O and stirred. Tothis was added N₂H₄ (0.076 mL, 2.36 mmol) in one portion. Upon addition,the solution lightened slightly in color to a brown-orange. The solutionwas allowed to stir for 15 min before the solution was concentrated to 5mL and layered with 5 mL of pentane. After 2 days at −35° C., darkorange crystals of [(TPB)Fe(N₂H₄)][BAr^(F) ₄] had formed (0.324 g, 89%).¹H NMR (C₆D₆/THF-d₈, δ): 53.72 (br s), 28.26 (s), 25.32 (s), 20.18 (brs), 8.28 (s, BAr^(F) ₄) 7.67 (s, BAr^(F) ₄), 8.14 (br s), 7.96 (br s),3.00 (br s), 2.67 (br s), 0.30 (br s), −26.06 (s). UV-Vis (THF) λ_(max),nm (ε, M⁻¹ cm⁻¹): 800 (140). Anal. Calc. for C₆₈H₇₀B₂F₂₄FeN₂P₃: C,52.98; H, 4.58; N, 1.82. Found: C, 53.03; H, 4.63; N, 1.70. Solutionmagnetic moment (THF-d⁸): 3.46 μ_(B).

Synthesis of [(TPB)Fe(NH₃)][BAr^(F) ₄], 4

A solution of 3 (0.308 g, 0.20 mmol) in 10 mL of 1:6 THF:Benzene wasrapidly stirred at RT for 12 h. After this time, the volatiles wereremoved in vacuo and the residue was taken up in Et₂O, filtered, andlayered with pentane before being cooled to −35°. After 2 days, darkorange-red crystals of [(TPB)Fe(NH₃)][BAr^(F) ₄] had formed (0.264 g,87%). ¹H NMR (C₆D₆/THF-d⁸, δ): 68.22 (br s), 28.55 (s), 24.28 (s), 17.81(br s), 8.34 (s, BAr^(F) ₄), 7.68 (s, BAr^(F) ₄), 5.74 (br s), 3.53 (s),2.15 (br s), 1.22 (br s), −25.48 (s). UV-Vis (THF) λ_(max), nm (ε, M⁻¹cm⁻¹): 871 (50). IR (KBr, cm-1): 3381 (v[NH]) Anal. Calc. forC₆₈H₆₉B₂F₂₄FeNP₃: C, 54.24; H, 4.55; N, 0.92. Found: C, 53.47; H, 4.72;N, 0.94. Solution magnetic moment (THF-d₈): 3.63 μ_(B).

Synthesis of (TPB)FeNH₂, 5

A solution of 2 (0.300 g, 0.20 mmol) in 5 mL of Et₂O was stirred overpowdered NaNH₂ (0.077 g, 1.99 mmol) for 1.5 h at room temperature. Overthis time, the solution darkened from orange to a dark brown. Volatileswere removed and the remaining residue was extracted with 40 mL ofpentane to yield a pale orange solution. To this solution was added12-crown-4 (0.070 g, 0.40 mmol) to aid in the removal of NaBAr^(F) ₄,and solids began to precipitate. The solution was allowed to stand for 1h before filtration. Removal of solvent for 3 h at 70° C. resulted in(TPB)FeNH₂ as a dark orange powder (0.060 g, 0.09 mmol, 46%). Crystalssuitable for X-ray diffraction were grown from slow evaporation of aconcentrated ethereal solution. Due to the presence of a small amount of12-crown-4 that had similar solubility properties to the product,satisfactory combustion analysis was not obtained for 5. ¹H NMR (C₆D₆,δ): 91.12 (br s), 38.21 (s), 25.42 (s), 4.12 (br s), 1.55 (br s), 0.21(br s), −3.04 (br s), −5.93 (br s), −20.19(s). UV-Vis (THF) λ_(max), nm(ε, M⁻¹ cm⁻¹): 700 (90), 930 (80). Solution magnetic moment (C₆D₆): 4.05μ_(B). We also wish to note that trace amounts (<3%) of the neutralcomplex (TPB)Fe(N₂) are typically detected by NMR spectroscopy inpreparations of 5. (TPB)Fe(N₂) and 5 also have similar solubilityproperties.

Synthesis of (TPB)FeOH, 6

2 (0.80 g, 0.05 mmol) was dissolved in 5 mL of Et₂O and stirred overNaOH (0.063 g, 1.6 mmol) at room temperature for 2 h during which thecolor of the solution darkened to a deep brown. Volatiles were removedfrom the solution and the resulting solids were extracted with pentaneto yield Synthesis of (TPB)FeOH (0.027, 77%) as a brown powder. Crystalssuitable for X-Ray diffraction were grown by a slow evaporation of aconcentrated Et₂O solution. ¹H NMR (C₆D₆, δ): 89.55 (br s), 39.07 (s),24.70 (s), 6.71 (s), 4.08 (s), 1.55 (br s), −0.52 (br s), −6.00 (br s),−21.02 (s). UV-Vis (THF) λ_(max), nm (ε, M⁻¹ cm⁻¹): 870 (230), 700(218). Anal. Calc. for C₃₆H₅₅BFeOP₃: C, 65.18; H, 8.36; N, 0. Found: C,65.15; H, 8.28; N, none found. Solution magnetic moment (C₆D₆): 4.12μ_(B).

Protonation of 5

A 20 mL scintillation vial was charged with 5 (0.005 g, 0.007 mmol) andHBAr^(F) ₄.2Et₂O (0.008 g, 0.007 mmol) and cooled to −35° C. 2 mL ofsimilarly cooled Et₂O was added to the mixture and the color of thesolution lightened rapidly. The solution was allowed to warm to roomtemperature over 30 minutes before volatiles were removed to yield 4(0.010 g, 0.006 mmol, 91%). The identity of the product was determinedvia ¹H NMR which was identical to that observed for 4.

Reduction of 4

A 20 mL scintillation vial was charged with 4 (0.025 g, 0.016 mmol) andKC₈ (0.0024 g, 0.018 mmol). 2 mL of Et₂O were added and the resultingdark suspension was allowed to stir for 2 h at RT. After this time, thesolution was filtered and volatiles were removed to yield (TPB)Fe(N₂) asa brown solid. The identity of the product was determined via ¹H NMRwhich was identical to the reported values for (TPB)Fe(N₂).

Monitored Conversion of 3 to 4

3 (0.020 g, 0.013 mmol) was dissolved in a 6:1 mixture of C₆D₆:THF-d₈.The resulting solution was transferred to an NMR tube equipped with acapillary containing a solution of (TPB)FeBr in a 6:1 mixture ofC₆D₆:THF-d₈ as an internal standard. This NMR tube was sealed with aJ-Young valve and was placed into a 500 MHz spectrometer which had beenpreheated to 60° C. The reaction was monitored via single scans everyminute for 4 hours during which time complete and clean conversion from3 to 4 was observed. After the reaction was complete, an aliquot of theheadspace was analyzed by GC for the presence of H₂. After this,volatiles were vacuum transferred onto a solution of HCl in THF. Afterthis, volatiles were removed and the resulting solids were diluted withwater to appropriate volumes to test for the presence of NH₃ via theindophenol test,⁸ or N₂H₄ with p-dimethylaminobenzaldehyde.⁹ Therelative amounts of products are compiled in Table 25 of this document.

Optimized coordinates [Å] for [(TPB)Fe]⁺:

Fe 0.00415000 −0.16241600 −0.77675900 P −2.16349100 −1.24719000−0.66738800 P 2.39121900 −0.83777200 −0.74771500 P −0.313454002.27645200 −0.74537300 C 2.09021700 −1.73435800 0.82972400 C 4.029524001.02631900 0.64266200 H 3.98567100 0.45071500 1.57275400 H 3.191040001.72365500 0.64140500 H 4.95767100 1.60939200 0.65520800 C −2.51735800−0.85749000 1.09566800 C −2.75787900 2.84164700 0.59302900 H −2.249721003.20855200 1.49066600 H −2.90111300 1.76680700 0.71127400 H −3.746215003.31410800 0.55282800 C 0.96713000 −1.34366200 1.61726000 C 0.682679001.41376900 1.69824000 C −3.77585900 −1.11913600 1.66212900 H −4.53614800−1.64204200 1.09100300 C −1.50541500 −0.21277600 1.85301900 C−1.85207300 0.20954700 3.15263700 H −1.11562400 0.74145000 3.74897600 C0.63534800 3.23145700 −2.06275600 H 0.57728600 4.28918000 −1.78194000 C1.88147700 2.83955400 3.29296500 H 2.40769300 2.94283300 4.23815900 C0.69310600 −2.13340400 2.75608400 H −0.16599700 −1.88793000 3.37259400 C4.03129200 0.10104900 −0.58072200 H 4.06604600 0.72216800 −1.48419000 C1.38189800 1.59489600 2.90786900 H 1.54195100 0.73989400 3.56048800 B0.03447200 −0.03107800 1.40854000 C 5.27977700 −0.79677000 −0.55921200 H6.17024200 −0.16055800 −0.49330400 H 5.38965200 −1.41148800 −1.45674600H 5.28761200 −1.45308000 0.31716600 C 1.72157800 3.94568000 2.45737900 H2.12008100 4.91463800 2.74408600 C 1.05459100 3.79624400 1.24157800 H0.95646000 4.65573100 0.58420600 C 2.77223500 −2.14130600 −2.05044100 H3.66239200 −2.68641700 −1.71685100 C −4.07155000 −0.72325700 2.96514500H −5.04800100 −0.93619400 3.39051300 C −3.10700900 −0.041891003.70690700 H −3.32936500 0.29181200 4.71691400 C 2.87573600 −2.838382001.19803000 H 3.71142600 −3.14386600 0.57548300 C −1.96911100 3.20280400−0.67407900 H −2.51737400 2.81757600 −1.54509900 C 1.62859900−3.14880400 −2.20719100 H 0.71343900 −2.65749200 −2.56047300 H1.40294400 −3.66587600 −1.27050000 H 1.89809900 −3.90564800 −2.95299300C 1.49436100 −3.21131000 3.13086000 H 1.25256900 −3.77768600 4.02619200C 2.11687300 2.84858500 −2.14500700 H 2.24202800 1.80944900 −2.46700100H 2.62947700 2.97816400 −1.18858400 H 2.61717700 3.48566700 −2.88385400C −2.49731300 −3.09116600 −0.81402900 H −3.54555800 −3.21968300−0.51323400 C 2.59216700 −3.56984800 2.34988300 H 3.21486600 −4.416104002.62556400 C −1.61976400 −3.87498400 0.17378600 H −1.85554000−4.94310500 0.10319500 H −1.78498100 −3.56065100 1.20749500 H−0.55428900 −3.75363600 −0.04648600 C 0.53094900 2.55057600 0.86165400 C−1.86943800 4.73224900 −0.80367000 H −2.87736400 5.16043800 −0.75057800H −1.42999600 5.06125000 −1.74899100 H −1.29175300 5.16729800 0.01900400C −2.95949200 −0.11225400 −3.13193300 H −2.81329100 −1.04459900−3.68700100 H −2.00927500 0.43395300 −3.13573300 H −3.686323000.48853300 −3.69112200 C −3.47778300 −0.36144900 −1.70257400 H−3.55367100 0.61070400 −1.20046300 C −2.34128800 −3.61958100 −2.24860700H −1.32591600 −3.46834000 −2.62965700 H −3.03896300 −3.15266000−2.94927700 H −2.53763200 −4.69785800 −2.26329900 C −0.047598003.05060900 −3.43027100 H −1.09647000 3.36299400 −3.42897900 H−0.00706500 2.00371300 −3.75748600 C 3.08823800 −1.45897600 −3.39314100H 3.31866300 −2.21769600 −4.14966000 H 3.94741000 −0.78399700−3.33422400 H 2.22923900 −0.88166100 −3.75845100 C −4.87834200−0.99390800 −1.73157800 H −5.54599600 −0.36236200 −2.32962300 H−4.87513300 −1.98675200 −2.19255400 H −5.32164500 −1.07767600−0.73669900 H 0.47189800 3.64859800 −4.18772700Optimized Coordinates [Å] for [(Me₂PhP)₃Fe]⁺:

Fe 0.15864700 −0.18495700 −0.99760500 P −0.96396300 1.83613500−1.46879900 P −0.45971000 −2.22271000 −0.05600500 P 2.48332300−0.04211700 −1.31271800 C −2.26951200 −2.22907300 0.27519800 C−1.24228800 2.66879600 0.14383800 C −2.79182200 −2.06800000 1.56749900 C4.03948300 −1.45884400 0.61288100 C −0.64552100 3.89539800 0.47185600 H−0.02266300 4.41711100 −0.24807900 C −2.04536400 2.02460300 1.10222300 C−2.25786700 2.60533100 2.35188400 H −2.89062600 2.10308200 3.07801800 C3.22714900 −1.29680000 −2.44619800 H 4.32141000 −1.26188800 −2.43248400C 4.38348500 −0.67980900 2.87994900 H 4.81234400 −0.82007700 3.86773800C −4.17301300 −2.01522100 1.77110700 H −4.56274900 −1.900889002.77861400 C 0.31804700 −2.68760000 1.54990900 H 1.38960600 −2.835603001.38660000 C 4.58567000 −1.63819900 1.88602900 H 5.17639900 −2.525196002.09670500 C 3.63159400 0.46273300 2.59776700 H 3.47437400 1.215628003.36480600 C 3.08377800 0.64390800 1.32807100 H 2.50486800 1.542439001.12392600 C −0.21389600 −3.73662900 −1.09026300 H −0.63896300−4.62100300 −0.60524200 C −0.85737200 4.47065600 1.72657200 H−0.39598100 5.42490200 1.96403800 C −1.66358000 3.82933600 2.66729400 H−1.83040700 4.28140300 3.64050500 C −3.16079100 −2.32427200 −0.80796000H −2.78261000 −2.44971700 −1.82004800 C 3.28247400 1.51522500−1.91488900 H 2.90698900 1.75077900 −2.91604500 C −5.04871600−2.11906600 0.69010800 H −6.12191000 −2.08279500 0.85179800 C−2.66656800 1.64342000 −2.16885400 H −3.18590300 2.60610400 −2.21168900C −4.53907500 −2.27482500 −0.60131300 H −5.21468500 −2.36443900−1.44718900 C 3.28537900 −0.31297600 0.31747500 C −0.23933400 3.11974200−2.57722600 H 0.77356200 3.37847300 −2.25945300 H 2.88637400 −1.08677000−3.46523900 H 2.89836600 −2.30593900 −2.18218500 H 3.02794700 2.34344400−1.24739500 H 4.37260400 1.42000600 −1.95477900 H 0.85730500 −3.89883800−1.24746400 H −0.68525200 −3.61298900 −2.06920800 H −0.11632500−3.60480000 1.96078100 H 0.20638200 −1.87762100 2.27591300 H −0.853927004.02578200 −2.60403500 H −0.18044400 2.70383300 −3.58796700 H−3.25191000 0.95080000 −1.55774000 H −2.59769400 1.23085800 −3.18107000H −2.51936000 1.07248800 0.87329200 H 4.21949800 −2.21267100 −0.14752200H −2.13022700 −1.99363600 2.42490400Mulliken Atomic Spin Densities for 2

1 Fe 3.253912 2 P −0.027171 3 P −0.010617 4 P −0.015937 5 C −0.004907 6C 0.000171 7 H 0.000020 8 H 0.000301 9 H 0.000144 10 C −0.002614 11 C0.000097 12 H −0.000011 13 H 0.000408 14 H −0.000066 15 C 0.031834 16 C0.029533 17 C 0.003234 18 H −0.000001 19 C 0.031179 20 C −0.010984 21 H0.000534 22 C 0.001306 23 H 0.000886 24 C 0.002732 25 H −0.000716 26 C−0.010913 27 H 0.000375 28 C −0.001689 29 H 0.000232 30 C −0.010241 31 H0.000537 32 B −0.267025 33 C −0.000540 34 H −0.000389 35 H 0.000017 36 H0.000034 37 C −0.004817 38 H 0.000268 39 C 0.002585 40 H 0.000043 41 C0.007248 42 H −0.000019 43 C −0.006843 44 H 0.000321 45 C 0.003235 46 H−0.000697 47 C 0.005544 48 H −0.000087 49 C −0.001772 50 H 0.000308 51 C0.002233 52 H −0.001672 53 H −0.000551 54 H 0.000270 55 C 0.006515 56 H−0.000827 57 C −0.000211 58 H 0.000126 59 H 0.000133 60 H −0.000055 61 C0.009143 62 H 0.000978 63 C −0.007547 64 H 0.000405 65 C −0.000028 66 H0.000024 67 H −0.000020 68 H −0.001178 69 C −0.002002 70 C 0.000981 71 H0.000333 72 H −0.000026 73 H −0.000056 74 C 0.001914 75 H −0.000172 76 H−0.000830 77 H 0.000312 78 C −0.004719 79 H 0.000585 80 C −0.000384 81 H−0.000422 82 H −0.000009 83 H −0.000029 84 C 0.000389 85 H −0.000118 86H −0.000747 87 C −0.000078 88 H 0.000160 89 H 0.000030 90 H −0.000011 91C −0.001228 92 H −0.000593 93 H 0.000076 94 H 0.000043 95 H −0.000115

Sum of Mulliken spin densities=3.00000

Simulation Parameters

Compound S g D (cm⁻¹) [TPB^(iPr)Fe][BAr^(F) ₄] 3/2 2.009 13.933[TPB^(iPr)Fe(N₂H₄)][BAr^(F) ₄] 3/2 2.031 19.937[TPB^(iPr)Fe(NH₃)][BAr^(F) ₄] 3/2 2.044 12.162 (TPB^(iPr))Fe(NH₂) 3/22.000 11.348

TABLE 25 Product quantification for the decomposition of 3 to 4 RunEquiv H₂ Equiv N₂H₄ Equiv NH₃ 1 0.01 None det. 0.09 2 0.009 None det.0.12 3 None det. None det. 0.14

TABLE 26 Crystal data and structure refinement for (TPB)FeMe (1)Identification code jsa200m Empirical formula C37 H57 B Cl Fe P3 Formulaweight 696.85 Temperature 293(2)K Wavelength 0.71073 Å Crystal systemTriclinic Space group P-1 Unit cell dimensions a = 10.9554(3) Å α =91.4010(10)°. b = 11.5075(3) Å β = 95.4060(10)°. c = 15.9312(4) Å γ =117.8130(10)°. Volume 1763.16(8) Å³ Z 2 Density (calculated) 1.313 Mg/m³Absorption coefficient 0.665 mm⁻¹ F(000) 744 Crystal size .456 × .304 ×.209 mm³ Theta range for 1.29 to 27.10°. data collection Index ranges−14 <= h <= 14, −14 <= k <= 14, −20 <= l <= 20 Reflections collected36496 Independent reflections 7758 [R(int) = 0.0311] Completeness to99.8% theta = 27.10° Refinement method Full-matrix least-squares on F²Data/restraints/parameters 7758/338/402 Goodness-of-fit on F² 1.086Final R indices R1 = 0.0321, wR2 = 0.0813 [l > 2sigma(l)] R indices (alldata) R1 = 0.0371, wR2 = 0.0860 Largest diff. peak and hole 0.890 and−0.305 e·Å⁻³

TABLE 27 Crystal data and structure refinement for [(TPB)Fe][BAr^(F) ₄](2) Identification code mem130 Empirical formula C68 H66 B2 F24 Fe N0 P3Si0 Formula weight 1509.59 Temperature 100(2)K Wavelength 0.71073 ÅCrystal system Orthorhombic Space group Pbca Unit cell dimensions a =26.4056(9) Å α = 90°. b = 19.7833(7) Å β = 90°. c = 26.4402(9) Å γ =90°. Volume 13812.1(8) Å³ Z 8 Density (calculated) 1.452 Mg/m³Absorption coefficient 0.393 mm⁻¹ F(000) 6168 Crystal size 0.32 × 0.30 ×0.26 mm³ Theta range for 1.85 to 33.73°. data collection Index ranges−35 <= h <= 41, −30 <= k <= 30, −41 <= l <= 41 Reflections collected379408 Independent reflections 27590 [R(int) = 0.0500] Completeness to100.0% theta = 33.73° Absorption correction Semi-empirical fromequivalents Max. and min. transmission 0.9046 and 0.8844 Refinementmethod Full-matrix least-squares on F² Data/restraints/parameters27590/18/951 Goodness-of-fit on F² 1.062 Final R indices R1 = 0.0496,wR2 = 0.1239 [l > 2sigma(l)] R indices (all data) R1 = 0.0744, wR2 =0.1400 Largest diff. peak and hole 0.740 and −0.532 e·Å⁻³

TABLE 28 Crystal data and structure refinement for[(TPB)Fe(N₂H₄)][BAr^(F) ₄] (3) Identification code jsa19_0m Empiricalformula C78 H68 B2 F24 Fe N2 P3 Formula weight 1659.72 Temperature296(2)K Wavelength 0.71073 Å Crystal system Orthorhombic Space groupPbca Unit cell dimensions a = 20.0031(7) Å α = 90°. b = 25.7862(8) Å β =90°. c = 26.6970(8) Å γ = 90°. Volume 13770.4(8) Å³ Z 8 Density(calculated) 1.601 Mg/m³ Absorption coefficient 0.404 mm⁻¹ F(000) 6776Crystal size 0.46 × 0.26 × 0.15 mm³ Theta range for 2.00 to 32.59°. datacollection Index ranges −30 <= h <= 30, −39 <= k <= 39, −40 <= l <= 40Reflections collected 347031 Independent reflections 25087 [R(int) =0.0696] Completeness to 99.9% theta = 32.59° Max. and min. transmission0.9419 and 0.8361 Refinement method Full-matrix least-squares on F²Data/restraints/parameters 25087/971/981 Goodness-of-fit on F² 0.937Final R indices R1 = 0.0531, wR2 = 0.1373 [l > 2sigma(l)] R indices (alldata) R1 = 0.0872, wR2 = 0.1681 Largest diff. peak and hole 1.295 and−0.755 e·Å⁻³

TABLE 29 Crystal data and structure refinement for[(TPB)Fe(NH₃)][BAr^(F) ₄] (4) Identification code xds_ascii Empiricalformula C68 H80 B4 F24 Fe N2 O2 P3 Formula weight 1605.34 Temperature293(2)K Wavelength 0.71073 Å Crystal system Orthorhombic Space groupPbca Unit cell dimensions a = 19.846(4) Å α = 90°. b = 25.821(5) Å β =90°. c = 26.862(5) Å γ = 90°. Volume 13765(5) Å3 Z 8 Density(calculated) 1.549 Mg/m³ Absorption coefficient 0.402 mm⁻¹ F(000) 6600Crystal size .25 × .15 × .15 mm³ Theta range for 1.50 to 25.18°. datacollection Index ranges −23 <= h <= 23, −30 <= k <= 30, −32 <= l <= 32Reflections collected 155016 Independent reflections 12232 [R(int) =0.0175] Completeness to 99.0% theta = 25.18° Refinement methodFull-matrix least-squares on F² Data/restraints/parameters 12232/972/961Goodness-of-fit on F² 1.041 Final R indices R1 = 0.0606, wR2 = 0.1680[l > 2sigma(l)] R indices (all data) R1 = 0.0620, wR2 = 0.1692 Largestdiff. peak and hole 1.741 and −1.346 e·Å⁻³

TABLE 30 Crystal data and structure refinement for (TPB)FeNH₂ (5)Identification code jsa23_0m Empirical formula C36 H56 B Fe N P3 Formulaweight 662.39 Temperature 100(2)K Wavelength 0.71073 Å Crystal systemTriclinic Space group P-1 Unit cell dimensions a = 10.9229(9) Å α =77.268(7)°. b = 11.2493(13) Å β = 84.862(5)°. c = 16.5084(15) Å γ =61.117(4)°. Volume 1732.1(3) Å³ Z 2 Density (calculated) 1.483 Mg/m³Absorption coefficient 0.615 mm⁻¹ F(000) 812 Crystal size 0.23 × 0.15 ×0.15 mm³ Theta range for 2.11 to 28.28°. data collection Index ranges−14 <= h <= 14, −14 <= k <= 14, −22 <= l <= 22 Reflections collected52847 Independent reflections 8561 [R(int) = 0.0706] Completeness to99.7% theta = 28.28° Max. and min. transmission 0.9134 and 0.8715Refinement method Full-matrix least-squares on F²Data/restraints/parameters 8561/2/397 Goodness-of-fit on F² 1.120 FinalR indices R1 = 0.0545, wR2 = 0.1236 [l > 2sigma(l)] R indices (all data)R1 = 0.0810, wR2 = 0.1324 Largest diff. peak and hole 1.541 and −0.646e·Å⁻³

TABLE 31 Crystal data and structure refinement for (TPB)FeOH (6)Identification code jsa21_0m Empirical formula C36 H56 B Fe N O P3Formula weight 678.39 Temperature 100(2)K Wavelength 0.71073 Å Crystalsystem Triclinic Space group P-1 Unit cell dimensions a = 10.9554(4) Å α= 77.466(2)°. b = 11.3311(4) Å β = 78.105(2)°. c = 16.6454(7) Å γ =61.338(2)°. Volume 1757.30(12) Å³ Z 2 Density (calculated) 1.282 Mg/m³Absorption coefficient 0.595 mm⁻¹ F(000) 726 Crystal size 10.00 × 0.29 ×0.17 mm³ Theta range for 2.07 to 37.78°. data collection Index ranges−18 <= h <= 17, −19 <= k <= 19, −28 <= l <= 28 Reflections collected142106 Independent reflections 18846 [R(int) = 0.0433] Completeness to100.0% theta = 37.78° Max. and min. transmission 0.9056 and 0.0661Refinement method Full-matrix least-squares on F²Data/restraints/parameters 18846/1/382 Goodness-of-fit on F² 1.024 FinalR indices R1 = 0.0316, wR2 = 0.0745 [l > 2sigma(l)] R indices (all data)R1 = 0.0477, wR2 = 0.0810 Largest diff. peak and hole 0.810 and −0.493e·Å⁻³

ADDITIONAL REFERENCES

-   ¹ W. Kabsch, J. Appl. Cryst. 1993, 26, 795.-   ² Sheldrick, G. M. Acta Cryst. 1990, A46, 467.-   ³ Sheldrick, G. M. Acta Cryst. 2004, A64, 112.-   ⁴ Müller, P. Crystallography Reviews 2009, 15, 57.-   ⁵ http://ewww.mpi-muelheim.mpg.de/baciloginsibill/julX_en.php-   ⁶ Gaussian 03, Revision E.01, M. J. Frisch, G. W. Trucks, H. B.    Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A.    Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M.    Millam, S. S. lyengar, J. Tomasi, V. Barone, B. Mennucci, M.    Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M.    Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.    Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E.    Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J.    Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.    Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A.    Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S.    Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K.    Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V.    Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B.    Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L.    Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A.    Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.    Chen, M. W. Wong, C. Gonzalez, and J. A. Pople, Gaussian, Inc.,    Wallingford Conn., 2004.-   ⁷ Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11,    3920-3922.-   ⁸ Weatherburn, M. W. Anal. Chem. 1967, 39, 971-974.-   ⁹ Watt, G. W.; Chrisp, J. D. Anal. Chem. 1952, 24, 2006-2008.

EXAMPLE 4 Comparative Activity of Phosphine Fe Complexes for NitrogenFixation

Three new Fe—N₂ ⁻ complexes supported by (o-Cy₂P(C₆H₄))₃B (TPB^(Cy)),(o-Ph₂P(C₆H₄))₃B (TPB^(Ph)), and (o-iPr₂P(C₆H₄))₂BPh (DPB) have beensynthesized and characterized. These complexes, along with the halidecomplex (TPB^(iPr))FeCl and the complexes [(PhBP^(iPr)₃)Fe(N₂)][MgCl(THF)₂] and (Cy₂P(C₂H₄))₃PFe(N₂) have been subjected toconditions to probe their efficacy for the reduction of N₂ to NH₃. Withthe exception of (PCy₂(C₂H₄))₃PFeN₂ all complexes evolve substantialquantities of NH₃ under the same conditions and[(TPB^(Cy))Fe(N₂)][Na(12-C-4)₂], [(TPB^(Ph))Fe(N₂)][Na(12-C-4)₂], and(TPB)FeCl are effective pre-catalysts for the reduction of N₂ to NH₃.Additionally, variations on reaction conditions with the catalystsystem, [(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂], have been investigated. Thesefindings further support the agency of a molecular species within thecatalytic cycle and offer insights into structural features and reactionconditions that enable catalytic turnover.

Introduction

Nitrogen fixation to NH₃ is a critical component of the global nitrogencycle.¹ This process is mediated by humans via the Haber-Bosch processwherein N₂ and H₂ are pressurized and heated over an Fe-based catalystto produce NH₃ on a massive scale.² Biologically, N₂ can be reduced toNH₃ by diazotrophs at cofactors that are rich in Fe and S but that canadditionally feature Mo or V.³ Due to the importance of N₂ fixation,there has been considerable attention devoted to understanding themechanism of this biological process.⁴ Molecular systems for thereduction of N₂ to NH₃ have traditionally focused on Mo centers due tothe early work of Chatt and Hidai on these systems as well as thepresence of Mo in the most thoroughly studied FeMo-cofactor.^(5,6)Indeed, the first well-defined molecular system competent for thereduction of N₂ to NH₃ at room temperature and pressure featured thetri-amido amine Mo systems of Schrock and co-workers^(7,8) while morerecently Nishibayashi and co-workers have reported only the second suchcatalytic system featuring a phosphine supported Mo system.⁹

Our lab^(10,11,12,13) and others^(14,15,16) have alternately beeninterested in Fe-mediated N₂ reduction, motivated by spectroscopic andbiological studies that suggest that Fe is the site of N₂ reduction toNH₃ in nitrogenases.^(17,18) Indeed, it has been shown that molecular Fespecies can split N₂ into two nitride units which can release nearlystoichiometric amounts of NH₃ upon acidification or hydrogenation andthat simple Fe precursors can catalytically generate N(TMS)₃ with strongreducing agents and TMSCl.^(13,20) With the hypothesis that a single Fecenter may be the site of N₂ reduction, we sought to synthesizethree-fold symmetric Fe complexes capable of supporting a variety ofnitrogenous ligands that could form en-route to NH₃ formation.Specifically, the utility of tris-phosphine borane supported Fe centersfor the binding of N_(x)H_(y) ligands and the functionalization of N₂suggest that such a single-site hypothesis bears merit.^(21,22,23) Thissummed research has recently culminated in the realization of an Febased system for the catalytic reduction of N₂ to NH₃.²⁴ When mixed with48 equivalents of HBAr^(F) ₄ (BAr^(F) ₄ ⁻=(3,5-(CF₃)₂C₆H₃)₄B⁻) and KC₈at −78° C. in Et₂O, the tris-phosphine borane supported[(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂] (TPB^(iPr)=(o-iPr₂P(C₆H₄))₃B,12-C-4=12-crown-4) forms 7 eq. of NH₃ per Fe center (Scheme 1).

Motivated by this result, we report the preparation of three newphosphine supported Fe complexes analogous to[(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂]: [(TPB^(Cy))Fe(N₂)][Na(12-C-4)₂] (1),[(TPB^(Ph))Fe(N₂)][Na(12-C-4)₂] (2), and [(DPB)Fe(N₂)][K(Bz15-C-5)₂] (3)(FIG. 81) and subject these complexes to the standard catalyticconditions used for [(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂]. For comparison thetris-phospine borate complex [(PhBP^(iPr) ₃)Fe(N₂)][MgCl(THF)₂] (4) hasalso been examined.²⁵ A new halide complex (TPB)FeCl (5) has also beensynthesized and examined for catalysis to test the effect of Cl⁻ onturnover. Finally, the known tetra-phosphine complex(Cy₂P(C₂H₄))₃PFe(N₂) (6) has been independently synthesized and has alsobeen subjected to the catalytic conditions.²⁶ Empirically, of thecomplexes studied, only species with flexible Fe—B interactions producesuper-stoichiometric quantites of NH₃ with[(TPB^(Cy))Fe(N₂)][Na(12-C-4)₂], [(TPB^(Ph))Fe(N₂)][Na(12-C-4)₂], and(TPB^(iPr))FeCl serving as pre-catalysts with 3.20, 2.20, and 3.16equivalents of NH₃ per Fe produced respectively. While these systems arenot as efficient as the parent [(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂], theynevertheless demonstrate that the ligand field supplied by the TPBligands is beneficial for catalytic turnover. In addition to thesestudies, further variations on the catalytic conditions for[(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂] such as temperature, acid, reductant,and solvent have been studied and suggest additional factors that may becrucial for catalytic turnover.

Results and Discussion

Ligand Synthesis: With the desire of investigating the structuralfeatures that enable catalysis, we targeted several variations on theTPB^(iPr) ligand scaffold. To probe the steric effects of the ligandscaffold we envisaged a cyclohexyl based TPB^(Cy) scaffold that wouldclosely mimic the electronics of the TPB^(iPr) system while providingadditional steric bulk. The TPB^(Ph) ligand also seemed like an obviousvariation on TPB^(iPr) to test the effect of weaker phosphine donors.²⁸Finally, our lab recently reported the synthesis of Fe complexes ofPhB(o-iPr₂P(C₆H₄)₂ (DPB).29,30,31 This DPB ligand ligates the Fe centerthrough two phosphine donors and replaces one phosphine donor with aninteraction through a BPh unit. The DPB scaffold hence provides a largervariation on the TPB scaffold than the simple change in phosphinesubstituent.

The preparation of TPB^(Ph) has already been reported²⁷ and thesynthesis of TPB^(Cy) (7) involves the lithiation of o-Br(C₆H₄)PCy2 andsubsequent reaction with ⅓ equivalents of BCl₃ analogously to thepreparation of TPB^(iPr).³² Metallation of the appropriatetris-phosphine ligands also proceeds in an analogous manner to thatreported for (TPB^(iPr))FeBr²¹ (Scheme 2) to yield the halides 5,(TBP^(Cy))FeCl (8), and (TPB^(Ph))FeCl (9) which are brown solids andall possess S=3/2 spin states as judged from their solution magneticmoments of μ_(eff)=3.8-4.0 μ_(B). Reduction of 8 with an excess of Na/Hgamalgam followed by addition of 12-C-4 results in formation of 1 as adark red S=½ complex with an N—N stretch at 1901 cm⁻¹, very close to thereported value for [(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂] (1905 cm⁻¹)suggesting that the N₂ molecule is similarly activated in both of thesecomplexes.

Reduction of phenyl substituted 9 with one equivalent of Na/Hg does notresult in the uptake of N₂ as is observed in the alkyl systems.Alternately, a diamagnetic species is obtained with ¹H NMR resonancesshifted into the olefinic region. These shifted peaks suggest thepossibility of an η⁶ coordinated aryl ring and the crystal structure(vide infra) confirms that one phosphine ligand has dissociated and oneof the phenyl groups attached to the phosphine is now coordinated to theFe center to give the bis-phosphino borane aryl complex (TPB^(Ph)′)Fe10. Further reduction of 10 with NaC₁₀H₈ and addition of 12-C-4 enablesisolation of the N₂ adduct 2 as a red solid. Complex 2 similarlydisplays a strong N—N stretching vibration in the IR at 1988 cm⁻¹ whichoccurs at substantially higher energy than that found in 1 or[(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂] reflecting the weaker donor ability ofthe tri-aryl phosphine donors. Finally, reduction of the[(DPB)Fe]₂(μ-1,2-N₂) with an excess of K/Hg amalgam followed by additionof Bz15-C-5 (Bz15-C-5=benzo-15-crown-5) results in dark red 3. The N—Nstretch for complex 3 is observed at 1935 cm⁻¹ suggesting that the DPBscaffold still results in a strongly activated N₂ despite the loss ofone phosphine donor.

Structural Characterization: Complexes 1, 3, 5, 8, 9, and 10 have beencrystallographically characterized and their structures are depicted inFIG. 82. The halide complexes 5, 8, and 9 display relatively similargeometries about Fe with relatively long Fe—P bonds consistent withother examples of formally Fe^(I) S=3/2 Fe centers from relatedcomplexes (Table 32).^(21,23) A trend in increasing Fe—P distances isobserved upon moving from complex 9 to 5 to 8 potentially arising fromthe increased steric demands from the substituents on phosphorus (Table32). The Fe—B distance in these halide complexes is also longer than thesum of the covalent radii of Fe and B (2.36 Å) likely indicating aminimal interaction between Fe and B. The geometry about Fe, consistentwith the long Fe—B distance, can best be described as pseudo-trigonalbipyramidal with a strong distortion towards tetrahedral as indicated byT ₄ values of <0.5 for all of the halide complexes.

The assignment of 10 as an η⁶ aryl adduct was confirmed by its XRDstructure which shows the Fe interacting with a phenyl ring and onephosphine ligand dissociated from Fe (Fe—P >3.5 Å). The Fe—C distancesall range from 2.095(3) Å to 2.172(3) Å with the ipso carbon being theclosest and an overall Fe-centroid distance of 1.595 Å. The other twoFe—P distances are substantially shorter than those observed in thehalide complexes, reflecting the diamagnetism of 10 (Table 32). Despitethe other short Fe ligand bonds, the Fe—B distance of 2.457(4) Å remainslong. The ΣC—B—C in 10, however, shows a value of 334° compared with anaverage of 341° in complexes 5, 8, and 9, indicating that the B is morepyramidalized in 10 and suggesting that there is more donation into thevacant p orbital on B in this complex than in the halide complexesdespite the long bond distance observed.

Although crystals of 2 suitable for XRD analysis could not be obtained,both complexes 1 and 3 were crystallographically characterized. Complex3 shows a pseudo tetrahedral geometry about Fe, consistent with othercomplexes of DPB,³⁰ and displays a strong Fe—C_(ipso) interaction of2.055(2) Å. In addition to the short Fe—C_(ipso) there is a moderatelyshortened Fe—C_(ortho) distance of 2.326(2) Å. The Fe—B distance of2.246(2) Å 3 is also short when compared with other DPB complexes of Fe.All of these bond metrics point to a strong interaction between theelectron rich Fe center and the BCC unit in 3, and this conclusion issupported by bond alteration in the bound Ph ring with two short C—Cdistances of 1.378(3) Å and 1.385(3) Å with the remainder of the C—Cdistances >1.408 Å. Finally, the Fe—N distance of 1.792(2) Å and the N—Ndistance of 1.135(3) Å can be compared with the analogous distances from[(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂] of 1.781(2) and 1.144(3) Å respectivelysuggesting a slightly weaker, but still significant, degree of N₂activation, consistent with the vibrational data for these complexes.

TABLE 32 Selected bond lengths (Å) and T₄ values for complexes 1, 3, 5,8, 9, and 10. Note that the values reported for 8 are the average offour molecules in the unit cell. Complex Fe-X Fe-B Fe-P1 Fe-P2 Fe-P3 T₄1 1.792(3) 2.292(3) 2.3157(9) 2.2228(9) 2.2219(9) 0.60 3 1.792(2)2.246(2) 2.2265(7) 2.2179(7) — — 5 2.2751(3) 2.4593(9) 2.4314(3)2.4063(3) 2.3792(3) 0.32 8 2.296 2.429 2.456 2.419 2.390 0.39 92.2712(4) 2.5418(16) 2.3622(4) 2.3451(4) 2.3325(4) 0.33 10 — 2.457(4)2.2466(10) 2.1846(9) — —

Complex 1 looks quite similar to [(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂] inoverall structure as both complexes have pseudo-trigonal bipyramidalgeometries with τ₄ values of 0.60 and 0.61 respectively. Both complexespossess one large P—Fe—P angle which is 136.21(3)° in 1 and 134.99(3)°in [(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂]. Unlike the corresponding halidecomplexes 8 and 5 which show longer Fe—P distances on average withcyclohexyl substituents, 1 displays very similar average Fe—P distancesto [(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂] with values of 2.254 and 2.251 Årespectively. The Fe—B distance is also essentially identical in both 1and [(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂] with a value of 2.292(3) Å in bothcomplexes. Finally, 1 has an Fe—N distance of 1.792(3) Å and a N—Ndistance of 1.143(4) Å, very close to the values observed in[(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂] and consistent with their similarvibrational features.

Catalytic Nitrogen Reduction Studies: With the above-mentioned complexesisolated and characterized, comparative studies of their efficacy aspre-catalysts were undertaken. Specifically targeted were complexes 1,2, 3, and 5 as well as the phosphine N₂ complexes 4 and 6. The resultsfrom these studies are summarized in Table 33. While none of theinvestigated catalysts outperform the original[(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂] system (7 eq. NH₃/Fe), several trendsrelevant to catalysis become apparent. Most notably, certain complexesare competent for catalytic N₂ reduction within this study, that iscomplexes that can on average produce >2 eq. NH₃/Fe, bear a great dealof structural similarity to [(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂].Specificially, of the new complexes, 1 differs from[(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂] only in a slightly larger stericprofile on the phosphine substituents. This is manifested in the similarv_(NN) frequencies and bond metrics for these two species. It istherefore not surprising that 1 also serves as a catalyst for N₂reduction. Complex 2 also serves as a pre-catalyst for N₂ reductionunder these standard conditions. While the Ph substituted ligandscaffold has substantially different electronic properties that the iPror Cy systems, the fact that it can still mediate catalysis suggeststhat the tris-phosphine borane ligand set common to all three catalystsis important to enable catalysis. Furthermore, the absence of anobservable Fe(0) N₂ complex in the Ph system might suggest that such aspecies is not required for catalysis, perhaps in the iPr or Cy systemsas well.

TABLE 33 N₂ reduction with phosphine Fe complexes. All data shown as anaverage of at least 3 runs (see SI) using the conditions described inthe experimental section. Entry Complex NH₃ eq./Fe 1[(TPB^(Cy))Fe(N₂)][Na(12-C-4)₂] 3.20 2 [(TPB^(Ph))Fe(N₂)][Na(12-C-4)₂]2.20 3 [(DPB)Fe(N₂)][K(Bz15-C-5)₂] 1.72 4 [(PhPB^(iPr)₃)Fe(N₂)][MgCl(THF)₂] 1.19 5 (TPB^(iPr))FeCl 3.16 6 (C₂H₄PCy₂)₃PFe(N₂)0.66

In contrast to the TPB based complexes, DPB based 3 does not produce >2eq. of NH₃/Fe on average. Even though 3 activates N₂ more strongly than2, the fact that it produces less NH₃ again suggests the importance ofthe tris-phosphine borane scaffold. Additionally, one might expect forthe DPB scaffold to be more labile than the TPB scaffold due to theweaker coordinating ability of the BPh unit. If the catalysis ismediated by a molecular species, this lability may compromise thestability of the species towards decomposition. The fact that 3 performsmore poorly than 1 or 2 suggests that such a pathway is not beneficialfor catalysis, circumstantially supporting the agency of a molecularspecies versus some heterogeneous species derived from decomposition.

It has already been noted that switching the apical atom of thetris-phosphine scaffold from B to Si results in loss of catalyticactivity, putatively due to the loss of the flexible Fe—B interaction.²⁴Similarly, when the B atom is tied back as a borate in complex 4, asub-stoichiometric amount of NH₃ which is again consistent with theempirical observation of a need for a hemi-labile interaction betweenthe Fe and the apical B. Interestingly, in complex 6 there exists, inprinciple, the possibility of a hemi-labile interaction with the apicalP atom of the ligand scaffold. The low yields of NH₃ for this systemwhen compared with the other entries in Table 33 suggest that if anyflexibility of the Fe—P bond exists, it does not enable catalysis inthis system. Finally, the halide complex 5 was subjected to the standardcatalytic conditions to probe the effect of chloride ion on catalysis.While diminished, the fact that complex 5 is a competent pre-catalystsuggests that any adventitious chloride contaminants, potentiallyarising from the HBAr^(F) ₄ acid, may hinder catalysis, but should notterminate a catalytic cycle.

The recent report of a C anchored complex, (CP^(iPr) ₃)FeN₂ ⁻,³³deserves special mention in this context, as this complex appears tohave more limited flexibility in the Fe—C bond when compared to the TPBscaffold yet still displays substantial catalytic activity. More studieswill be required to provide a satisfying explanation for thisobservation, but possibilities include the requirement of a light atomin the backbone of the ligand for catalytic turnover or access to adifferent mechanism of reduction in the C based system.

TABLE 34 Variations on the standard catalytic conditions with[(TPB^(iPr))Fe(N₂)] [Na(12-C-4)₂] as pre-catalyst. Note that allreported values are an average of at least 2 runs (See SI). (a) Resultsin reference 24. (b) 10% Na by weight. Entry Variation NH₃ eq./FeSolvent  1 iPr₂O 6.53  2 DME 3.70  3 Bu₂O 3.16  4 Toluene 0.78  5 1:6Et₂O:Toluene 3.12 Reductant   6^(a) Cp*₂Co 0.6   7^(a) Cp*₂Cr <0.2  8^(a) K metal 0.4   9^(b) Na/Hg 2.08 10 NaC₁₀H₈ 1.00 11 MgC₁₄H₁₀ 0.28Acid  12^(a) HOTf 0.4  13^(a) HCl <0.1  14^(a) [Lutidinium][BAr^(F) ₄]<0.1 15 [2,6-dimethylanilinium][OTf] 2.1 16[2,6-dimethylanilinium][BAr^(F) ₄] 2.9 Temperature 17 −110° C. 5.40 18^(a)    25° C. 1.33

Several additional experiments were performed in an effort to furtherexplore the reaction space for catalysis. Table 34 lists variations onsolvent, reductant, acid, and temperature. While none of theseconditions led to an improvement in catalysis, they do reveal some ofthe trends in conditions that enable catalytic turnover. As one cangather from entries 1-5, the solvent scope for catalysis is limited torelatively non-polar ethereal solvents. Unsurprisingly, iPr₂O servesequally well as Et₂O in the reaction, but moving to a more polar solventin DME results in a substantial drop in NH₃ yield, likely due enhancedreactivity between the reductant and acid. Furthermore, the presence ofsome ethereal solvent is important, likely due to the need to dissolvethe HBAr^(F) ₄ acid, as is evidenced by the low yields of NH₃ in toluene(entry 4), compared with much higher yields when even a small amount ofEt₂O is included in the reaction mixture (entry 5). The lower yield ofNH₃ in Bu₂O is likely similarly explained by lowered solubility ofHBAr^(F) ₄.

Aside from solvent, the reaction in some embodiments is sensitive to thechoice of reductant. Although other evidence supports the presence of amolecular catalyst, some sort of graphite bound complex as thecatalytically active species cannot be ruled out. Entry 6 illustratesthat Na/Hg is also competent for catalysis, albeit only nominally,suggesting that graphite is not an essential component of the reactionmixture. The strong homogeneous reductant NaC₁₀H₈ yielded substantial,but not catalytic, quantities of NH₃ and MgC₁₄H₁₀ provided relativelylow yields of NH₃. The lower yields of NH₃ with the homogeneousreductants Cp*₂Co and Cp*₂Cr may be due to their more positivepotentials relative to the other reductants canvassed. These morepositive potentials should not allow for formation of[(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂] and the lowered efficacy of thesereductants perhaps indicates the need to form[(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂] under turnover.

Various acids have also been investigated for their efficacy inpromoting turnover (Table 34, entries 12-16). While HBAr^(F) ₄.2 Et₂O isa strong acid and functions well for turnover, other strong acids inHOTf and HCl result in lower yields of NH₃. The acid used by bothSchrock and Nishibayashi, [Lutidinium][BAr^(F) ₄], also does not resultin catalytic turnover, but switching to other nitrogen based acids in[2,6-dimethylanilinium][OTf] or [2,6-dimethylanilinium][BAr^(F) ₄] doesresult in catalytic NH₃ formation. This result is somewhat surprising,as the pKa values for these acids are comparable with [Lutidinium]having a pK_(a) of 6.77 and [2,6-dimethylanilinium] having a pK_(a) of3.95.^(34,35) The cause of the disparate reactivity between these twoacids is not clear, but the stark difference may indicate a requiredpK_(a) for turnover that is bracketed by the values for the two acids inquestion.

Finally, while the initial reaction screening was carried out at −78°C., we desired to further test the dependence of turnover ontemperature. Previously, we had reported that catalytic runs at roomtemperature resulted in significantly lowered yields of NH₃ (entry18).²⁴ In contrast, we wanted to determine whether the catalysis wascompatible with even lower temperatures. Interestingly, the turnover isstill viable at temperatures as low as −110° C. (entry 10). Although wecannot exclude the possibility that reaction in this case only occurredupon warming in these runs, the conversion of the bronze color of KC₈ toblack graphite in the cold reaction mixture suggests reaction at thesetemperatures may be occurring. Furthermore, attempts to quench thereaction at −78° C. with either [TBA][CN] or t-BuNC still resulted inthe formation of 3.50 and 1.67 equivalents of NH₃ respectively. Thelowered activity suggests that quenching is occuring, but that somecatalysis had already occurred at low temperature. These results arealso consistent with catalytic activity at very low temperatures.

CONCLUSIONS

These combined studies varying both the pre-catalyst and the reactionconditions for the Fe-mediated reduction of N₂ to NH₃ provide severalkey insights on the catalysis. Firstly, all evidence still supports theagency of a molecular species. The results are generally consistent witha molecular species as the active catalyst. Furthermore, the flexibleFe—B linkage in the TPB scaffold is empirically important for catalysis.Although other ligand scaffolds with light atoms in the axial positionare viable for turnover, it is unclear whether a differing mechanismenables catalysis in this system. Additionally, turnover isunsurprisingly sensitive to the exact ligand set employed as well as thereaction conditions. While all of the conditions surveyed showed adecrease in catalytic activity relative to our original report, severalvariations on the reaction conditions did preserve catalytic activity,providing motivation that alternate conditions to improve the catalysismay be available. Indeed, even slight steric variation in the ligand setor in the choice of solvent seems to have a significant impact on thecatalysis. Taken together, the studies presented here delineate theconditions and structures that are required for catalytic turnover.

Experimental

General Considerations. Unless otherwise noted, all compounds werepurchased from commercial sources and used without further purification.Complexes 4²⁵ and 6²⁶ as well as [(TPB^(iPr))Fe(N₂)][Na(12-C-4)₂],²¹[(DPB)Fe]₂(μ-1,2-N₂),³⁰ MgC₁₄H₁₀, TPB^(iPr),³² TPB^(Ph),²⁷2-Cy₂PC₆H₄Br,³⁶ and HBAr^(F) ₄.2 Et₂O³⁷ were prepared according toliterature procedures. All manipulations were carried out under an N₂atmosphere utilizing standard glovebox or Schlenk techniques. Solventswere dried and de-oxygenated by an argon sparge followed by passagethrough an activated alumina column purchased from S.G. Waters Company.Solvents for catalytic runs were additionally stirred for more than 2hours over Na/K alloy then filtered prior to use.

IR spectra were obtained via KBr pellets on a Bio-Rad Excalibur FTS 3000spectrometer using Varian Resolutions Pro software set at 4 cm⁻¹resolution. Alternately, IR spectra were obtained on a Bruker Alpha ATRspectrometer on solid samples. NMR measurements were obtained on Varian300 MHz or 500 MHz spectrometers. Deuterated solvents for thesemeasurements were obtained from Cambridge Isotope Laboratories and weredried and degassed prior to use. All ¹H NMR spectra were referenced toresidual solvent peaks while ³¹P NMR measurements were referenced to anexternal standard of H₃PO₄ and ¹¹B NMR spectra were referenced to anexternal standard of Et₂O.BF₃. UV-Visible spectra were taken on a Cary50 spectrometer from 1100 nm to 200 nm in the fast scan mode. Sampleswere prepared in a 1 cm path length quartz cuvette. All samples had ablank sample background subtraction applied.

XRD data was obtained at low temperatures on a Siemens or BrukerPlatform three-circle diffractometer coupled to a Bruker-AXS Smart ApexCCD detector with graphite-monochromated Mo Kα radiation (λ=0.71073),performing φ-and ω-scans. Data for complex 4 was collected on withsynchrotron radiation at the Stanford Synchrotron Radiation Laboratory(SSRL) beam line 12-2 at 17 keV using a single phi axis and recorded ona Dectris Pilatus 6M. The images were processed using XDS³⁸ and furtherworkup of the data was analogous to the other datasets. All structureswere solved by standard direct or Patterson methods and refined againstF² using the SHELX program package.^(39,40,41) All atoms, with theexception of hydrogens, have been anisotropically refined. The hydrogenatoms bonded to atoms of interest, namely N or O, have been located inthe difference map and refined semi-freely. All other hydrogen atomswere included via a standard riding model.

Catalytic runs and NH₃ quantification were performed in a manneridentically to that previously reported.²⁴ Ammonia quantification wasalso performed as previously described.⁴² The only exception being thata large excess of reductant was used in Na/Hg (10% by weight) runs(0.280 g, 0.140 mmol). Procedural details and results for all catalyticruns can be found infra.

TPB^(Cy), 7. A schlenk tube with a stir bar was filled with a solutionof 2-dicyclohexylphosphinebromobenzene (1.000 g, 2.830 mmol) in toluene(10 mL) and sealed. The schlenk tube was then hooked up to a schlenkline under a stream of N₂ and the Teflon stopper of the schlenk tube wasreplaced with a rubber septum. The reaction vessel was then cooled to−78° C. A 1.6 M solution of n-BuLi in hexane (1.77 mL, 2.830 mmol) wasthen slowly added via syringe. The resulting solution turned slightlyorange while stirring for 15 min at −78° C. The solution was then warmedto ambient temperature and stirred for an additional hour before beingcooled back to −78° C. At this time a 1 M solution of BCl₃ in heptane(0.92 mL, 0.920 mmol) was added to the stirring reaction vessel. Thepale orange color of the solution lightened slightly upon addition ofthe BCl₃. The septa was exchanged with a Teflon stopcock and thesolution was allowed to stir at −78° C. for 1 hour before being warmedto room temperature and stirred for an additional 2 hours during whichtime a brown color developed in the solution. The mixture was thenheated to 90° C. for 16 hours during which time the brown colorlightened to orange and solids precipitated. Solvent was removed invacuo and the remaining waxy solid was extracted 3× with Et₂O (10 mL)and filtered. The remaining pale orange solution was concentrated tohalf volume and cooled to −35° C. for 16 hours which resulted in theformation of white crystals of the title compound (0.426 g, 0.513 mmol,55%). ¹H NMR (C₆D₆, δ): 8.30 (br s, 4H), 7.46 (br s, 4H), 7.24 (br s,2H), 7.18 (m, 2H), 1.93 (br m, 12H), 1.71 (br m, 12H), 1.63 (br s, 12H),1.36 (br s, 8H), 1.17 (br m, 22H). ³¹P{¹H} NMR (C₆D₆, δ): −2.54 (br s).¹¹B{¹H} NMR (C₆D₆, δ): 25.68 (vbr s). ¹³C{¹H} NMR (C₆D₆, δ): 159.69 (brs), 136.12 (br s), 131.97 (s), 127.30 (s), 35.83 (s), 31.29 (br s),30.27 (s), 27.86 (d, J=20 Hz), 27.01 (s).

(TPB^(iPr))FeCl, 5. A mixture of FeCl₂ (0.087 g, 0.69 mmol), TPB^(iPr)(0.400 g, 0.69 mmol), Fe powder (0.415 g, 7.40 mmol), and THF (20 mL)was heated to 90° C. in a sealed schlenk tube under vigorous stirringfor 3 days, during which time the color of the liquid phase turned frompale yellow to brown. The solids were removed from the mixture byfiltration, and the solvent was removed in vacuo. The brown residue wasthen triturated then extracted with pentane (200 mL) and filteredthrough celite to give a brown solution. Solvent evaporation in vacuoafforded the product as a greenish brown powder (0.422 g, 90%). Ananalytically pure sample was obtained by slow concentration of asaturated pentane solution. Crystals suitable for XRD analysis wereobtained upon cooling a saturated solution of 5 in pentane to −35° C. ¹HNMR (C₆D₆, δ): 96.93 (br s), 35.00 (s), 23.59 (s), 9.77 (br s), 5.76(s), 1.89 (br s), 1.62 (sh), −0.24 (s), −2.22 (br s), −22.39 (s). UV-vis(THF, nm {cm⁻¹M⁻¹}): 280 {2.0·10⁴}, 320 {sh}, 560 {sh}, 790 {150}, 960{190}. μ_(eff) (C₆D₆, Evans method, 20° C.): 4.0 μ_(B). Anal. calcd. forC₃₆H₅₄BClFeP₃: C, 63.41; H, 7.98; found: C, 63.16; H, 7.72.

(TBP^(Cy))FeCl, 8. A mixture of FeCl₂ (0.076 g, 0.60 mmol), 7 (0.500 g,0.60 mmol), Fe powder (0.333 g, 6.02 mmol), and THF (10 mL) was heatedto 90° C. in a sealed schlenk tube under vigorous stirring for 3 days,during which time the color of the liquid phase turned from pale yellowto brown. The solids were removed from the mixture by filtration, andthe solvent was removed in vacuo. The brown residue was then trituratedthen extracted with pentane (200 mL) and filtered through celite to givea brown solution. Solvent evaporation in vacuo afforded the product as agreenish brown powder (0.460 g, 83%). An analytically pure sample andcrystals suitable for XRD analysis were obtained by slow concentrationof a saturated pentane solution. ¹H NMR (C₆D₆, δ): 81.75 (br s), 36.12(s), 25.66 (s), 17.65 (br s), 5.01 (s), 3.77 (br s), 1.85 (s), 1.56 (brs), 0.14 (s), −0.13 (s), −0.58 (s), −0.74 (sh), −1.54 (s), −1.97 (br s),−3.87 (br s), −5.92 (br s), −7.12 (br s), −23.01 (s). UV-vis (THF, nm{cm⁻¹ M⁻¹}): 560 {130}, 750 {100}, 950 {150}. μ_(eff) (C₆D₆, Evansmethod, 20° C.): 3.8 μ_(B), Anal. calcd. for C₅₄H₇₈BClFeP₃: C, 70.33; H,8.52; found: C, 70.45; H, 8.49.

(TPB^(Ph))FeCl, 9. A Schlenk tube was charged with TPB^(Ph) (0.923 g,1.240 mmol), FeCl₂ (0.198 g, 1.560 mmol) and Fe powder (0.176 g, 3.160mmol) and THF (50 mL). The reaction was stirred vigorously for 3 days at70° C., during which time the slurry turned dark brown. The mixture wasfiltered through celite to remove the excess iron powder and thevolatiles removed in vacuo. The residual solids were triturated withtoluene, slurried in CH₂Cl₂ and filtered to collect a dark brown powder(0.841 g, 76%). Crystals suitable for X-ray analysis were grown by slowconcentration of a C₆H₆ solution. ¹H NMR (d₈-THF, δ): 34.23, 23.92,9.90, 7.30, 4.73, 2.31, −23.84. μ_(eff) (d₈-THF, Evans method, 25° C.):4.0 μ_(B). We were unable to obtain satisfactory elemental analysis.

(TPB^(Ph))Fe, 10. Sodium (0.003 g, 0.148 mmol) and mercury (0.500 g)were stirred vigorously with C₆H₆ (1 mL). A slurry of (TPB^(Ph))FeCl(0.095 g, 0.107 mmol) in C₆H₆ (10 mL) was added and the reaction mixturestirred for 6 hours at room temperature. The resulting dark red mixturewas filtered and lyophilized (0.080 g, 88%). Crystals suitable for X-rayanalysis were grown by vapor diffusion of diethyl ether into aconcentrated THF solution at −35° C. ¹H NMR (C₆D₆, δ): 8.38 (t, J=8.4Hz, 2H), 7.94 (d, J=7.4 Hz, 1H), 7.85 (t, J=7.3 Hz, 2H), 7.59 (ddt,J=9.6, 7.3, 3.1 Hz, 4H), 7.41 (d, J=7.5 Hz, 1H), 7.31-7.17 (m, 6H),7.09-6.79 (m, 12H), 6.73 (dd, J=4.4, 2.6 Hz, 3H), 6.60 (t, J=8.6 Hz,2H), 6.54-6.42 (m, 2H), 6.33-6.15 (m, 2H), 6.04 (t, J=5.9 Hz, 1H), 4.42(q, J=6.7 Hz, 1H), 4.16 (q, J=5.6 Hz, 1H), 3.89 (d, J=6.1 Hz, 1H), 3.62(q, J=6.6 Hz, 1H). ¹³C{¹H} NMR (C₆D₆, δ): 160.5 (s), 145.7 (d, J=7.0Hz), 144.1 (s), 143.8 (s), 142.9 (s), 142.5 (s), 141.9 (s), 141.5 (s),140.9 (s), 140.6 (s), 139.8 (s), 139.7 (s), 137.7 (s), 137.4 (s), 136.0(d, J=10.8 Hz), 135.6 (s), 135.2 (m), 134.8 (s), 134.6 (s), 132.9 (m),132.3 (d, J=8.7 Hz), 132.0 (d, J=7.3 Hz), 130.9 (m), 129.2 (s), 129.0(s), 128.7 (s), 126.9 (m), 126.3 (d, J=8.7 Hz), 125.9 (s), 124.3 (s),123.8 (d, J=6.6 Hz), 123.6 (d, J=6.5 Hz), 109.2 (d, J=15.7 Hz), 94.0 (d,J=14.3 Hz), 88.6 (s), 87.0 (s), 85.8 (s), 85.5 (m). ³¹P{¹H} NMR (C₆D₆,δ): 85.65 (d, J=81.8 Hz), 69.65 (d, J=82.2 Hz), −12.92 (s). ¹¹B{¹H}(C₆D₆, δ): 15.9. Anal. calcd. for C₅₄H₄₂BFeP₃: C, 76.26; H, 4.98. Found:C, 76.69; H, 5.59.

[(TPB^(Cy))Fe(N₂)][Na(12-C-4)₂], 1. Sodium (0.030 g, 1.304 mmol) andmercury (1.0 g) were mixed in a vial with a stir bar. Complex 8 (0.137g, 0.150 mmol) was dissolved in THF (10 mL) and added to the freshlyprepared Na/Hg amalgam. The resulting mixture was vigorously stirred for1 hour during which time the color of the solution changed from darkbrown to a deep red. The solution was then filtered through celite andvolatiles were removed in vacuo to yield a red residue. This residue wastaken up in Et₂O (10 mL) and again filtered through celite to provide adark red solution. 12-C-4 (0.052 g, 0.297 mmol) was added and theresulting solution was allowed to sit over which time the productprecipitated as red crystals. ¹H NMR (THF-d₈, δ): 18.48 (vbr s), 12.50(br s), 10.02 (s), 7.97 (vbr s), 7.04 (br s), 5.90 (br s), 5.51 (br s),4.22 (br s), 3.79 (s, [Na(12-C-4)₂]), 2.12 (br s), 1.65 (br s), 1.50 (brs), 0.96 (vbr s), 0.56 (br s), 0.30 (br s), 0.06 (br s), −1.11 (vbr, s),−3.71 (vbr s). IR (ATR, solid): v_(NN)=1905 cm⁻¹. UV-vis (THF, nm {cm⁻¹M⁻¹}): 500 {sh}, 850 {40}. μ_(eff) (THF-d₈, Evans method, 20° C.): 1.6μ_(B). Anal. calcd. for C₇₀H₁₁₀BFeN₂NaO₈P₃: C, 65.16; H, 8.59; N, 2.17;found: C, 64.89; H, 8.57; N, 2.21.

[(TPB^(Ph))Fe(N₂)][Na(12-C-4)₂], 2. Naphthalene (0.005 g, 0.039 mmol)was weighed into a vial with Na (0.01 g, 0.435 mmol), THF (2 mL) and astir bar. The resulting mixture was then vigorously stirred for 2 hoursover which time the liquid phase became dark green. The resultingsolution of NaC₁₀H₈ was then filtered through a glass fiber filter paperinto a −35° C. solution of 10 (0.030 g, 0.035 mmol) in THF (2 mL) whichrapidly changed color from orange to dark red. The resulting solutionwas stirred for an additional 10 minutes before volatiles were removed.The dark red residue was washed 3× with pentane (2 mL) and thenextracted into Et₂O (10 mL) and filtered through celite. The resultingred solution was then treated with 12-C-4 (0.013 g, 0.074 mmol) and afine red powder immediately precipitated. The mixture was allowed tostand for 1 hour before the supernatant was decanted and the resultingred solids were washed 3× with Et₂O (2 mL) to yield the title compoundas a red powder (0.029 g, 66%). ¹H NMR (THF-d₈, δ): 11.61 (br s), 9.75(br s), 8.66 (s), 8.42 (s), 6.96 (vbr s), 6.03 (br s), 3.66 (s,[Na(12-C-4)₂]), 0.35 (vbr s), −0.07 (s), −0.80 (vbr s). IR (ATR, solid):vNN=1988 cm⁻¹. UV-vis (THF, nm {cm⁻¹ M⁻¹}): 500 {sh}. μ_(eff) (THF-d₈,Evans method, 20° C.): 1.7 μ_(B). Complex 2 is unstable to prolongedvacuum which precluded satisfactory combustion analysis.

[(DPB)Fe(N₂)][K(Bz15-C-5)₂], 3. A solution of [(DPB)Fe]₂(μ-1,2-N₂)(0.025 g, 0.023 mmol) and K/Hg amalgam (1 g, 1% K by weight) in THF (1mL) was stirred at RT for 4 hr. The dark red solution was decanted andfiltered through glass fiber filter paper onto solid Bz15-C-5 (0.026 g,0.098 mmol). Solvent was removed in vacuo and the resulting solids werewashed 3× with Et₂O (1 mL) and C₆H₆ (3×1 mL) to give dark solids of 3(0.048 g, 0.042 mmol, 90%). Single crystals were grown by layering a THFsolution with Et₂O and pentane. ¹H NMR (THF-d₈, δ) 14.50, 12.05, 6.86(C), 6.73 (C), 3.85 (C), 3.72 (C), 2.80, 1.01, −2.72, −4.78. IR (ATR,thin film): v_(NN)=1935 cm⁻¹. Anal. calcd. for C₅₈H₈₁BFeKN₂O₁₀P₂: C,61.43; H, 7.20; N, 2.47; found: C, 60.09; H, 7.33; N, 1.71.

[2,6-dimethylanilinium][OTf]. 2,6-dimethylaniline (0.500 g, 4.1 mmol)was dissolved in pentane (10 mL). This solution was then cooled to −35°C. before HOTf (0.619 g, 4.1 mmol) was added to the solution withstirring. Upon addition of HOTf, white precipitate formed, and theresulting suspension was allowed to warm to RT and was stirred for 1hour. After this time, the solids were allowed to settle before thesupernatant was decanted off. The solids were then dried under vacuumand subsequently washed 3× with pentane (5 mL) and 3× with Et₂O (5 mL)to yield the title compound as a white powder (0.894 g, 3.30 mmol, 80%).¹H NMR (10:1 CDCl₃:DMSO-d₆, δ) 7.06 (vbr s), 6.84 (m, 3H), 2.14 (s, 6H).¹³C NMR (10:1 CDCl₃:DMSO-d₆, δ) 131.6 (s), 129.1 (s), 128.2 (s), 128.0(s), 17.5 (s). Anal. calcd. for C₉H₁₂F₃NO₃S: C, 39.85; H, 4.46; N, 5.16;found: C, 39.84; H, 4.37; N, 4.90.

[2,6-dimethylanilinium][BAr^(F) ₄].1 Et₂O. 2,6-dimethylaniline (0.036 g,0.296 mmol) was dissolved in Et₂O (3 mL). To this was added a solutionof HBAr^(F) ₄.2 Et₂O (0.300 g, 0.296 mmol) in Et₂O (3 mL) and theresulting clear solution was allowed to stir for 1 hour. After thistime, the solution was concentrated to 3 mL and layered with pentane (3mL) and cooled to −35° C. for 3 days, over which time white crystals ofthe product formed (0.200 g, 0.182 mmol, 62%). ¹H NMR (10:1CDCl₃:DMSO-d₆, δ) 7.65 (s, 8H), 7.5 (vbr s, 3H), 7.48 (s, 4H), 7.11 (m,1H), 7.04 (m, 2H), 3.43 (q, J=7 Hz, 4H), 2.30 (s, 6H), 1.15 (t, J=7 Hz,6H). ¹³C NMR (10:1 CDCl₃:DMSO-d₆, δ) 161.5 (q), 134.5 (s), 131.1 (s),129.4 (s), 128.6 (m), 125.7 (s), 123.0 (s), 120.2 (s), 117.3 (s), 65.6(s), 17.5 (s), 15.0 (s). Anal. calcd. for C₄₄H₃₄BF₂₄NO: C, 49.88; H,3.23; N, 1.32; found: C, 49.77; H, 3.03; N, 1.24.

REFERENCES

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EXAMPLE 5 Comparison of Molecular Co and Fe Complexes in the CatalyticConversion of N₂ to NH₃

Abstract

The synthesis of a series of tris(phosphino)borane-ligated Co complexesis reported. The reactivity of these species with proton and electronsources in the presence of N₂ at −78° C. is described, and notably[(TPB)Co(N₂)][Na(12-crown-4)₂] (TPB=[o-(^(i)Pr₂P)C₆H4]₃B) is found tomediate the formation of 2.4 equivalents of NH₃/Co under theseconditions.

It is well understood that the ability of a molecular system to activatebound dinitrogen (N₂) relates to the propensity of that system tomediate functionalization of the N₂ moiety. This principle is evidenced,for example, in the early reports that Mo(0) and W(0) N₂ complexes(v_((N-N)) 2025-1910 cm⁻¹) generate NH₃ and N₂H₄ upon treatment withacid,¹ whereas for instance HCo(N₂)(PPh₃)₃ (one of the earliest reportedtransition metal dinitrogen complexes,² v_((N-N)) 2088 cm⁻¹) onlyquantitatively releases N₂ upon treatment with acid, with no evidencefor N₂ functionalization.^(1d,3) Furthermore, this cobalt complex can bereductively deprotonated to generate more activated Co—N₂ complexes,such as [(PPh₃)₃Co(N₂)][Li(Et₂O)₃] (v_((N-N)) 1900 cm⁻¹), which upontreatment with acid do produce some NH₃ and N₂H₄ (in this case 0.21 and0.22 equivalents respectively).³ With this principle in mind extensiveefforts have been made to study the activation and functionalization ofN₂ bound to metal centers of varying electronic properties.⁴ Indeed insome extreme cases systems have been shown to activate bound N₂ to theextent that the N—N bond is fully cleaved.^(5,6) Furthermore, in manycases it is has been shown that treatment of strongly activated N₂complexes with acid or H₂ leads to some reduced nitrogenproducts.^(1,3,6) However, this guiding principle alone has beeninsufficient to lead to many synthetic species capable of catalyticconversion of N₂ to NH₃. In this regard it is prudent to study the fewsystems known to catalyze this reaction with an emphasis on identifyingimportant properties aside from their degree of bound N₂ activation.⁷⁻⁹Understanding these principles may lead to the design of bettersynthetic catalysts as well as providing clues as to how nitrogenaseenzymes might operate.

For instance, it has recently been discovered that atris(phosphino)borane-ligated Fe complex is capable of catalyzing theconversion of N₂ to NH₃ at −78° C.⁸ It has been further postulated thatthe success of this system in activating N₂ stoichiometrically andmediating its catalytic conversion to NH₃ may arise from importantgeometric variability at the metal center provided by a highly flexibleFe—B interaction.^(10,11) Such geometric variability is proposed toallow a single Fe center to access different coordination environments,alternately stabilizing π-acidic or π-basic nitrogenous moieties thatmay be sampled along a N₂ fixation pathway.⁶ Consistent with thishypothesis, the isostructural (P₃X)-ligated Fe systems have been studiedand found to have a drastic dependence of activity on the identity ofthe X atom: with the least flexible X=Si system furnishing divergentlylow NH₃ yields, whereas the more flexible X=C or B systems affordmoderate yields of NH₃.^(8,9) However, the X=Si precursor also displaysweaker N₂ activation than the X=C or B species (vide infra), allowingfor an alternate hypothesis whereby the importance of the M-Xinteraction is to modulate the π-basicity of the metal center.

In an attempt to differentiate these hypotheses, we sought to explorethe activity of a species displaying comparably high metal-ligandflexibility but weak N₂ activation. Our previous studies have shown thattris(phosphino)borane-ligated Co complexes possess a flexible Co—Binteraction.¹² In addition, they only weakly activate N₂ and weretherefore excellent candidates as a model platform for this study. Forcomparison with the isostructural (TPB)Fe series whose N₂ fixationactivity we have described,⁸ we decided to target an analogous redoxseries of N₂ complexes with Co supported by the TPB ligand.

The (TPB)Co(N₂) complex (Scheme 1, 1) provided a logical entry point tothe desired redox series. The cyclic voltammagram of 1 in THF displays abroad feature corresponding to an oxidation process at 0.2 V vs. Fc/Fc⁺and a quasi-reversible reduction wave at −2.0 V vs. Fc/Fc⁺. Prompted bythis result, we explored the one electron chemical reduction andoxidation of 1. Treatment of 1 with 1 equivalent of NaC₁₀H₈ followed by2 equivalents of 12-crown-4 generates diamagnetic[Na(12-crown-4)₂)][(TPB)Co(N₂)] as red crystals (Scheme 1, 2).

The v_((N-N)) stretch of 2 is lower in energy than that of 1, 1978 and2089 cm⁻¹ respectively, and the solid state structure of 2 displayscontracted Co—N, Co—B, and Co—P distances compared to 1, consistent withincreased backbonding to each of these atoms. The one-electron oxidationof 1 can be achieved by addition of 1 equivalent of [H.(OEt₂)₂][BAr^(F)₄] at low temperature followed by warming, which generates red-purple[(TPB)Co][BAr^(F) ₄] (Scheme 1, 3, BAr^(F)₄=tetrakis(3,5-bistrifluoromethlyphenyl)borate). SQUID magnetometrymeasurements indicate that 3 adopts a high spin (S=1) state in the solidstate with no evidence for spin crossover. The structure of 3 confirmsthat [(TPB)Co][BAr^(F) ₄] does not bind N₂ in the solid state. The lackof dinitrogen binding at room temperature for 3 is consistent with thebehavior of the isostructural Fe complex, [(TPB)Fe][BAr₄ ^(F)].¹⁴

An assessment of the degree of Co—B bond flexibility of the (TPB)Coscaffold can be made by comparing the metal-boron interatomic distancesof the halide species (TPB)CoBr and the N₂ anion 2, an analogouscomparison can be made for the isostructural (TPB)Fe complexes. In the(TPB)Fe series, the Fe—B distance decreases upon reduction and N₂coordination from 2.458(5) Å in (TPB)FeBr to 2.293(3) Å in[Na(12-crown-4)₂)][(TPB)Fe(N₂)] (Δ_(M-B)=0.165 Å).¹¹ In the (TPB)Coseries, the Co—B distance also decreases upon reduction and N₂coordination from 2.4629(8) Å in (TPB)CoBr¹² to 2.301(3) Å in 2(Δ_(M-B)=0.162 Å). This evidence suggests that the (TPB)Co platformexhibits a similar degree of axial ligand flexibility to that observedfor the (TPB)Fe system. Furthermore, the N₂ ligand of 2 is lessactivated, as evidenced by vibrational spectroscopy, than the analogous(TPB)Fe, (CP₃)Fe, or (SiP₃)Fe complexes whose relative N₂ fixationactivity we have described (FIG. 1, SiP₃=[o-(^(i)Pr₂P)C₆H₄]₃Si,CP₃=[o-(^(i)Pr₂P)C₆H₄]₃C). This is as expected for the generally lessπ-basic metal center, attributable to the less spatially diffused-orbitals of Co.

The reactivity of these (TPB)Co complexes with sources of protons andelectrons in the presence of N₂ was investigated. In analogy to the[Na(12-crown-4)₂][(TPB)Fe(N₂)] complex, treatment of a suspension of 2in Et₂O at −78° C. with excess [H.(OEt₂)₂][BAr^(F) ₄] followed by excesspotassium graphite (KC₈) under an atmosphere of N₂ leads to theformation of 2.4±0.3 equivalents of NH₃ (120% per Co, average of 6iterations, Equation 1). No hydrazine is observed. It is worth notingthat each of the 6 experimental iterations under the standard conditionsprovided yields of ≧2.1 equivalents of NH₃ per Co and the mostproductive iteration yielded 2.8 equivalents of NH₃ per Co. Thesenominally superstoichiometric yields are consistent with modestcatalytic N₂ conversion by a subpopulation of Co complexes. Notably, noammonia is formed when either 2, [H.(OEt₂)₂][BAr^(F) ₄], or KC₈ isomitted from the standard conditions, indicating that all threecomponents are necessary for NH₃ production. These results, combinedwith our previous studies of N₂ fixation by (P₃X)M complexes,demonstrate that the degree of N₂ activation by the (SiP₃)Fe catalyticprecursor is not solely responsible for the divergently low N₂conversion activity of that system (FIG. 83). Indeed, [Na(12-crown-4)₂][(SiP₃)Fe(N₂)] shows a higher degree of N₂ activation than 2, yet 2demonstrates higher N₂ conversion activity.

TABLE 35 Ammonia Generation by Co Complexes Under the Standard ReactionConditions Entry Co complex NH₃ equiv/Co A[(TPB)Co(N₂)][Na(12-crown-4)₂]^(a) (2) 2.4 ± 0.3 B (TPB)Co(N₂) (1) 0.8 ±0.3 C [(TPB)Co][BAr^(F) ₄] (3) 1.6 ± 0.2 D (TPB)CoBr 0.7 ± 0.4 E(SiP₃)Co(N₂) <0.1 F [(NArP₃)CoCl][BPh₄] <0.1 G (DPB)Co(N₂) 0.3 ± 0.1 H(PBP)Co(N₂) 0.4 ± 0.2 I Co(PPh₃)₂I₂ 0.4 ± 0.1 J CoCp2 0.1 ± 0.1 KCo₂(CO)₈ <0.1 ^(a)Average of 6 iterations. All other yields are reportedas an average of 3 iterations.

Interestingly, though anionic 2 and cationic 3 both generatedsubstantial NH₃ under the standard conditions, submitting neutral 1 tothese conditions provided attenuated yields of NH₃, comparable to theyields obtained with (TPB)CoBr (Table 35, A-D). The effect of varyingthe X atom in the (P₃X) scaffold complexes of Co was also investigated.In this context, the known X=Si complex¹⁵ (Table 35, E) and a previouslyunreported X=N Co complex¹⁶ (Table 35, F) were subjected to the standardreaction conditions. Of the (P₃X)Co complexes screened, only(TPB)-ligated Co complexes generated ≧0.5 equivalents of NH₃ per metalcenter, underscoring again the importance of the nature of the M-Xinteraction in facilitating N₂ fixation by complexes of this type. Tofurther explore the generality of the observed reactivity for Cocomplexes, additional Co precursors were submitted to the standardreaction conditions. The precursors examined consisted of abis(phosphino)borane-ligated Co—N₂ complex¹⁷ (Table 35, G), abis(phosphino)boryl Co—N₂ complex¹⁸ (H), as well as various simple Cocomplexes (I-K). Again none of the precursors screened provided ≧0.5equivalents of NH₃ per metal center.

To conclude, we have described a series of tris(phosphine)borane-ligatedCo complexes and demonstrated the ability of a molecular Co-dinitrogencomplex to facilitate the conversion of N₂ to NH₃ at −78° C. in thepresence of proton and electron sources (2.4 equivalents of NH₃generated per Co center on average). Importantly, the propensity of the(P₃X)M complexes we have studied to perform productive nitrogen fixationappears not to depend solely on the ability of the precursor complex toactivate bound N₂. This observation provides further support for thehypothesis that the ability of these complexes to mediate the conversionof N₂ to NH₃ arises from important geometric variability at the metalcenter provided by a flexible M-X interaction. This report alsorepresents the first example of superstoichiometric N₂ to NH₃ conversionby a Co-dinitrogen complex.

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All manipulations were carried out using standard Schlenk or gloveboxtechniques under an N₂ atmosphere. Solvents were deoxygenated and driedby thoroughly sparging with N₂ followed by passage through an activatedalumina column in a solvent purification system by SG Water, USA LLC.Nonhalogenated solvents were tested with sodium benzophenone ketyl intetrahydrofuran in order to confirm the absence of oxygen and water.Deuterated solvents were purchased from Cambridge Isotope Laboratories,Inc., degassed, and dried over activated 3-Å molecular sieves prior touse.

[H.OEt₂][BAr^(F) ₄],¹ KC₈,² (TPB)Co(N₂),³ (TPB)CoBr,³ (SiP₃)Co(N₂),⁴NArP₃,⁵ (PBP)Co(N₂),⁶ DPB,⁷ and Co(PPh₃)₂|₂ ⁸ were prepared according toliterature procedures. All other reagents were purchased from commercialvendors and used without further purification unless otherwise stated.Et₂O for NH₃ generation reactions was stirred over Na/K (≧2 hours) andfiltered before use.

Physical Methods:

Elemental analyses were performed by Midwest Microlab, LLC(Indianapolis, Ind.). ¹H and ¹³C chemical shifts are reported in ppmrelative to tetramethylsilane, using ¹H and ¹³C resonances from residualsolvent as internal standards. ³¹P chemical shifts are reported in ppmrelative to 85% aqueous H₃PO₄. Solution phase magnetic measurements wereperformed by the method of Evans.⁹ IR measurements were obtained as thinfilms formed by evaporation of solutions using a Bruker Alpha PlatinumATR spectrometer with OPUS software. Optical spectroscopy measurementswere collected with a Cary 50 UV-vis spectrophotometer using a 1-cmtwo-window quartz cell. Electrochemical measurements were carried out ina glovebox under an N₂ atmosphere in a one compartment cell using a CHInstruments 600B electrochemical analyzer. A glassy carbon electrode wasused as the working electrode and platinum wire was used as theauxiliary electrode. The reference electrode was Ag/AgCl in THF. Theferrocene couple (Fc⁺/Fc) was used as an internal reference. THFsolutions of electrolyte (0.3 M tetra-n-butylammoniumhexafluorophosphate, TBAPF₆) and analyte were also prepared under aninert atmosphere.

X-ray Crystallography:

X-ray diffraction studies were carried out at the Caltech Division ofChemistry and Chemical Engineering X-ray Crystallography Facility on aBruker three-circle SMART diffractometer with a SMART 1K CCD detector.Data was collected at 100K using Mo Kα radiation (λ=0.71073 Å).Structures were solved by direct or Patterson methods using SHELXS andrefined against F2 on all data by full-matrix least squares withSHELXL-97.20 All non-hydrogen atoms were refined anisotropically. Allhydrogen atoms were placed at geometrically calculated positions andrefined using a riding model. The isotropic displacement parameters ofall hydrogen atoms were fixed at 1.2 (1.5 for methyl groups) times theUeq of the atoms to which they are bonded.

Notes Specific to Individual Structures:

The 12-crown-4 fragments are disordered for[Na(12-crown-4)₂][(TPB)Co(N₂)]. One 12-crown-4 (with O atoms labeledO5-O8 and O50-O80) is fully disordered over two unique positions. Theother 12-crown-4 (O atoms labeled O1-O4) shows a disorder in the methylcarbons but not the oxygen atoms. In all cases, the positions of thecarbons could be located in the difference map and refinedanisotropically and the hydrogen atoms were placed at geometricallycalculated positions as usual. The oxygen atom in one of the solvent THFmolecules (O10 and O100) is disordered over two positions. The other THFmolecule (with O9) shows large thermal ellipsoids, potentiallyindicating an unresolved disorder of this moiety. A diethylethermolecule was located on an inversion center and is therefore disorderedabout this symmetry element. The occupancies of all disordered fragmentswere freely refined and the bond lengths and angles were restrained tobe the same for the disordered fragments. Hydrogen atoms were notincluded on any of the solvent molecules for these reasons.

(DPB)Co(N₂) crystallizes with two monometallic (DPB)Co(N₂) complexes andone half of a bimetallic [(DPB)Co(N₂)]₂ complex in each asymmetric unit.This was also observed for the isostructural complex (DPB)Ni(N₂).¹⁰ Forthe bimetallic fragment, one of the isopropyl substituents on P1 isdisordered in which the methine carbon is disordered over two positions(C7A and C7B) and one of the methyl carbons is disordered over twopositions (C8A and C8B). The occupancies of the disordered fragmentswere freely refined and the bond lengths and angles were restrained tobe the same for the disordered fragments. As such, hydrogen atoms werenot included on this isopropyl substituent.

The fluorine substituents on C143, C144, C152, and C168 in[(TPB)Co][BAr₄ ^(F)] are disorded by rotation about the C—C bonds tovarying extents and were refined as two-part positional disorders ineach case. The occupancies of the disordered fragments were freelyrefined and the bond lengths and angles were restrained to be the samefor the disordered fragments.

[Na(12-crown-4)₂][(TPB)Co(N₂)] (2): To a −78° C. solution of (TPB)CoBr(70.5 mg, 0.0967 mmol) in THF (2 mL) was added a freshly preparedsolution of NaC₁₀H₈ (23.5 mg C₁₀H₈, 0.222 mmol) in THF (3 mL). Thesolution was brought to RT and allowed to stir for six hours. Additionof 12-crown-4 (51.1 mg, 0.290 mmol) and removal of solvent in vacuoprovided a dark red solid. Et₂O was added and subsequently removed invacuo. The residue was suspended in C₆H₆ and filtered and the solidswere washed with C₆H₆ (2×2 mL) and pentane (2×2 mL) to furnish a redsolid (68.8 mg, 0.0660 mmol, 68%). Single crystals were grown by vapordiffusion of pentane onto a THF solution of the title compound that hadbeen layered with Et₂O. NMR peaks are somewhat broadened likely owing tothe presence of a small amount of (TPB)Co(N₂). ¹H NMR (400 MHz, THF-d₈)δ 7.41 (3H), 6.94 (3H), 6.66 (3H), 6.44 (3H), 3.64 (32H), 2.29 (br),1.37 (6H), 1.20 (6H), 0.93 (6H), −0.26 (6H). ¹¹B NMR (128 MHz, THF-d₈) δ9.32. ³¹P NMR (162 MHz, THF-d₈) δ 62.03. IR (thin film, cm⁻¹): 1978(N₂). Anal. Calcd. for C₅₂H₈₆BCoN₂NaO₈P₃: C, 59.32; H, 8.23; N, 2.66.Found: C, 59.05; H, 7.99; N, 2.47.

[(TPB)Co][BAr^(F) ₄] (3): To a −78° C. solution of (TPB)Co(N₂) (91.5 mg,0.135 mmol) in Et₂O (2 mL) was added solid [H(OEt₂)₂][BAr^(F) ₄] (134.0mg, 0.132 mmol). The reaction was brought to RT and vented to allow forthe escape of H₂. The purple-brown solution was stirred for 1 hr. Thesolution was layered with pentane (5 mL) and stored at −35° C. tofurnish red-purple single crystals of the title compound (162.9 mg,0.0952 mmol, 82%) which were washed with pentane (3×2 mL). ¹H NMR (400MHz, C₆D₆) δ 26.25, 23.80, 8.64, 8.44 ([BAr^(F) ₄]), 7.88 ([BAr^(F) ₄]),6.33, −2.16, −3.68. UV-Vis (Et₂O, nm {L·cm⁻¹·mol⁻¹}): 585 {1500}, 760{532}. Anal. Calcd. for C₆₈H₆₆B₂CoF₂₄P₃: C, 53.99; H, 4.40. Found: C,53.94; H, 4.51.

(DPB)Co(N₂): A solution of DPB (0.8483 g, 1.79 mmol) and CoCl₂ (0.2316g, 1.78 mmol) in THF (ca. 80 mL) was stirred at RT until all of thesolids dissolved, leaving a deep blue solution. Solvent was removed invacuo and rated in Et₂O for 15 min. Solvent was removed in vacuo and theblue residue was dissolved in benzene (80 mL). Freshly prepared 1%sodium mercury amalgam (0.0864 g Na, 3.76 mmol) was added to the darkblue solution and stirred vigorously for 24 hours. The resulting darkred-orange solution was decanted and filtered through celite.Lyophilization of the decanted benzene solution leaves a fine dark brownpowder. This powder was dissolved in minimal Et₂O and cooled in afreezer to −30° C. affording red-orange crystals of (2). (0.850 g, 1.51mmol, 84.8%) Solution magnetic moment (Evans method, RT, C₆D₆): 1.8μ_(B). IR (thin film, cm⁻¹): 2098 (N₂). ¹H NMR (C₆D₆, 300 MHz, 23° C.):δ 17.79, 14.75, 4.05, 2.40, −2.72, −4.04; UV-Vis (toluene, nm {M⁻¹cm⁻¹}): 288 {9350}. Elemental analysis shows low values for N consistentwith a labile N₂ ligand Calcd. for C₅₂H₈₆BCoN₂NaO₈P₃: C, 64.19; H, 7.36;N, 4.99. Found: C, 64.52; H, 7.57; N, 4.13.

[(NArP₃)CoCl][BPh₄]: THF (5 mL) was added to a solid mixture of NArP₃(58 mg, 91.2 mmol), CoCl₂ (12 mg, 92.4 mmol) and NaBPh₄ (31 mg, 90.6mmol). The reaction was stirred for 4 hours at room temperature duringwhich the color evolved from yellow to green to purple. The solvent wasremoved in vacuo and the residue was taken in dichloromethane. Thesuspension was filtered over a plug of Celite and the filtrate was driedyielding a purple powder (86 mg, 82.1 mmol, 90%). Single crystals weregrown by slow evaporation of a saturated solution of [(NArP₃)CoCl][BPh₄]in diethyl ether/dichloromethane (1:2 v:v). ¹H NMR (CD₂Cl₂, 300 MHz, 23°C.): δ 177.77, 37.50, 23.78, 13.48, 12.96, 7.37, 7.08, 6.92, 4.41, 1.50,−3.60, −9.81; UV-Vis (THF, nm {L·cm⁻¹·mol⁻¹}): 564 {452}, 760 {532};μ_(eff) (CD₂Cl₂, Evans method, 23° C.): 3.97 μ_(B). Anal. Calcd. forC₆₃H₈₀BClCoNP₃: C, 72.10; H, 7.68; N, 1.33. Found: C, 71.97; H, 7.76; N,1.30.

Ammonia Quantification: A Schlenk tube was charged with HCl (3 mL of a2.0 M solution in Et₂O, 6 mmol). Reaction mixtures were vacuumtransferred into this collection flask. Residual solid in the reactionvessel was treated with a solution of [Na][O-t-Bu] (40 mg, 0.4 mmol) in1,2-dimethoxyethane (1 mL) and sealed. The resulting suspension wasallowed to stir for 10 min before all volatiles were again vacuumtransferred into the collection flask. After completion of the vacuumtransfer, the flask was sealed and warmed to room temperature. Solventwas removed in vacuo, and the remaining residue was dissolved in H₂O (1mL). An aliquot of this solution (20 μL) was then analyzed for thepresence of NH₃ (present as [NH₄][Cl]) by the indophenol method.¹¹Quantification was performed with UV-vis spectroscopy by analyzingabsorbance at 635 nm.

Standard NH₃ Generation Reaction Procedure with[(TPB)Co(N₂)][Na(12-crown-4)₂] (2): [(TPB)Co(N₂)][Na(12-crown-4)₂] (2.2mg, 0.002 mmol) was suspended in Et₂O (0.5 mL) in a 20 mL scintillationvial equipped with a stir bar. This suspension was cooled to −78° C. ina cold well inside of a N₂ glovebox. A solution of [H.(OEt₂)₂][BAr^(F)₄] (95 mg, 0.094 mmol) in Et₂O (1.5 mL) similarly cooled to −78° C. wasadded to this suspension in one portion with stirring. Residual acid wasdissolved in cold Et₂O (0.25 mL) and added subsequently. This mixturewas allowed to stir 5 minutes at −78° C., before being transferred to aprecooled Schlenk tube equipped with a stir bar. The original reactionvial was washed with cold Et₂O (0.25 mL) which was added subsequently tothe Schlenk tube. KC₈ (16 mg, 0.119 mmol) was suspended in cold Et₂O(0.75 mL) and added to the reaction mixture over the course of 1 minute.The Schlenk tube was then sealed, and the reaction was allowed to stirfor 40 min at −78° C. before being warmed to room temperature andstirred for 15 min.

TABLE 36 UV-vis quantification results for standard NH₃ generationexperiments with [(TPB)Co(N₂)][Na(12-crown-4)₂] (2) Iteration Absorbance(635 nm) Eq. NH₃/Co % Yield Based on H⁺ A 0.225 2.3 16 B 0.187 2.1 14 C0.199 2.2 14 D 0.240 2.5 18 E 0.255 2.8 19 F 0.197 2.2 14 Average 0.217± 0.027 2.4 ± 0.3 16 ± 2 Hydrazine was not detected in the catalyticruns using a standard UV-Vis quantification method.¹²Standard NH₃ Generation Reaction Procedure with (TPB)Co(N₂) (1):

The procedure was identical to that of the standard NH₃ generationreaction protocol with the changes noted. The precursor used was(TPB)Co(N₂) (1.3 mg, 0.002 mmol).

TABLE 37 UV-vis quantification results for standard NH₃ generationexperiments with (TPB)Co(N₂) (1) Iteration Absorbance (635 nm) Eq.NH₃/Co % Yield Based on H⁺ A 0.064 0.7 4 B 0.058 0.6 4 C 0.107 1.2 8Average 0.076 ± 0.027 0.8 ± 0.3 5 ± 2Standard NH₃ Generation Reaction Procedure with [(TPB)Co][BAr^(F) ₄](3):

The procedure was identical to that of the standard NH₃ generationreaction protocol with the changes noted. The precursor used was[(TPB)Co(N₂)][BAr^(F) ₄] (2.3 mg, 0.002 mmol).

TABLE 38 UV-vis quantification results for standard NH₃ generationexperiments with [(TPB)Co][BAr^(F) ₄] (3) Iteration Absorbance (635 nm)Eq. NH₃/Co % Yield Based on H⁺ A 0.092 1.4 6 B 0.122 1.8 9 C¹ 0.091 1.56 Average 0.107 ± 0.021 1.6 ± 0.2 7 ± 1 ¹Used 2.0 mg (0.001 mmol) ofcatalyst; omitted from average absorbanceStandard NH₃ Generation Reaction Procedure with (TPB)CoBr:

The procedure was identical to that of the standard NH₃ generationreaction protocol with the changes noted. The precursor used was(TPB)CoBr (1.6 mg, 0.002 mmol).

TABLE 39 UV-vis quantification results for standard NH₃ generationexperiments with (TPB)CoBr Iteration Absorbance (635 nm) Eq. NH₃/Co %Yield Based on H⁺ A 0.035 0.3 2 B 0.101 1.0 7 C¹ 0.088 0.7 6 Average0.068 ± 0.047 0.7 ± 0.4 5 ± 3 ¹Used 2.0 mg (0.003 mmol) of catalyst;omitted from average absorbanceStandard NH₃ Generation Reaction Procedure with (SiP₃)Co(N₂):

The procedure was identical to that of the standard NH₃ generationreaction protocol with the changes noted. The precursor used was(SiP₃)Co(N₂) (1.5 mg, 0.002 mmol).

TABLE 40 UV-vis quantification results for standard NH₃ generationexperiments with (SiP₃)Co(N₂) Iteration Absorbance (635 nm) Eq. NH₃/Co %Yield Based on H⁺ A <0.005 <0.1 — B <0.005 <0.1 — C <0.005 <0.1 —Average — <0.1 —Standard NH₃ Generation Reaction Procedure with [(NArP₃)CoCl][BPh₄]:

The procedure was identical to that of the standard NH₃ generationreaction protocol with the changes noted. The precursor used was[(NArP₃)CoCl][BPh₄] (1.9 mg, 0.002 mmol).

TABLE 41 UV-vis quantification results for standard NH₃ generationexperiments with [(NArP₃)CoCl][BPh₄] Iteration Absorbance (635 nm) Eq.NH₃/Co % Yield Based on H⁺ A <0.005 <0.1 — B <0.005 <0.1 — C <0.005 <0.1— Average — <0.1 —Standard NH₃ Generation Reaction Procedure with (DPB)Co(N₂):

The procedure was identical to that of the standard NH₃ generationreaction protocol with the changes noted. The precursor used was(DPB)Co(N₂) (1.4 mg, 0.002 mmol).

TABLE 42 UV-vis quantification results for standard NH₃ generationexperiments with (DPB)Co(N₂) Iteration Absorbance (635 nm) Eq. NH₃/Co %Yield Based on H⁺ A 0.033 0.24 2 B 0.036 0.28 2 C¹ 0.032 0.38 2 Average0.035 ± 0.002 0.3 ± 0.1 2 ± 0.2 ¹Used 0.9 mg (0.0016 mmol) of catalyst;omitted from average absorbanceStandard NH₃ Generation Reaction Procedure with (PBP)Co(N₂):

The procedure was identical to that of the standard NH₃ generationreaction protocol with the changes noted. The precursor used was(PBP)Co(N₂) (1.1 mg, 0.002 mmol).

TABLE 43 UV-vis quantification results for standard NH₃ generationexperiments with (PBP)Co(N₂) Iteration Absorbance (635 nm) Eq. NH₃/Co %Yield Based on H⁺ A 0.021 0.15 1 B 0.03  0.29 2 C 0.057 0.62 4 Average0.036 ± 0.019 0.4 ± 0.2 2 ± 1Standard NH₃ Generation Reaction Procedure with Co(PPh₃)₂|₂:

The procedure was identical to that of the standard NH₃ generationreaction protocol with the changes noted. The precursor used wasCo(PPh₃)₂|₂ (1.8 mg, 0.002 mmol).

TABLE 44 UV-vis quantification results for standard NH₃ generationexperiments with Co(PPh₃)₂I₂ Iteration Absorbance (635 nm) Eq. NH₃/Co %Yield Based on H⁺ A¹ 0.036 0.3 2 B 0.036 0.3 2 C 0.046 0.4 3 Average0.041 ± 0.007 0.4 ± 0.1 2 ± 0.4 ¹Used 2.0 mg (0.0024 mmol) of catalyst;omitted from average absorbanceStandard NH₃ Generation Reaction Procedure with CoCp₂:

The procedure was identical to that of the standard NH₃ generationreaction protocol with the changes noted. The precursor used was CoCp₂(0.6 mg, 0.003 mmol).

TABLE 45 UV-vis quantification results for standard NH₃ generationexperiments with CoCp₂ Iteration Absorbance (635 nm) Eq. NH₃/Co % YieldBased on H⁺ A 0.020 0.09 1 B 0.008 0.02 0 C 0.033 0.20 2 Average 0.020 ±0.013 0.1 ± 0.1 1 ± 1Standard NH₃ Generation Reaction Procedure with Co₂(CO)₈:

The procedure was identical to that of the standard NH₃ generationreaction protocol with the changes noted. The precursor used wasCo₂(CO)₈ (0.4 mg, 0.001 mmol, 0.002 mmol Co) sampled as a 100 μL aliquotof a stock solution (2.0 mg Co₂(CO)₈ in 0.5 mL Et₂O).

TABLE 46 UV-vis quantification results for standard NH₃ generationexperiments with Co₂(CO)₈ Iteration Absorbance (635 nm) Eq. NH₃/Co %Yield Based on H⁺ A <0.005 <0.1 — B <0.005 <0.1 — C <0.005 <0.1 —Average — <0.1 —

TABLE 47 Crystal data and structure refinement for[Na(12-crown-4)₂][(TPB)Co(N₂)] (2) and [(TPB)Co][BAr^(F) ₄] (3)Identification code 2 3 Empirical formula C₆₂H₈₆BCoN₂NaO_(10.5)P₃C₆₈H₆₆B₂CoF₂₄P₃ Formula weight 1212.97 1512.67 Temperature/K 100 (2) 100(2) Crystal system monoclinic orthorhombic Space group P2₁/n Pbca a/Å10.8142 (5) 26.3920 (15) b/Å 27.5046 (13) 19.7049 (13) c/Å 22.3660 (10)26.4995 (19) α/° 90 90 β/° 91.141 (2) 90 γ/° 90 90 Volume/Å³ 6651.2 (5)13781.1 (16) Z 4 8 ρ_(calc) g/cm³ 1.211 1.458 μ/mm⁻¹ 0.391 0.424 F(000)2576 6176 Crystal size/mm³ 0.38 × 0.30 × 0.25 0.35 × 0.32 × 0.24Radiation MoKα (λ = 0.71073) MoKα (λ = 0.71073) 2⊖ range for datacollection/° 3.94 to 86.26 3.72 to 64.06 Index ranges −20 ≦ h ≦ 20, −52≦ k ≦ 52, −39 ≦ h ≦ 39, −23 ≦ k ≦ 29, −43 ≦ l ≦ 43 −39 ≦ l ≦ 39Reflections collected 451328 377520 Independent reflections 49547[R_(int) = 0.0632, 23962 [R_(int) = 0.0539, R_(sigma) = 0.1797]R_(sigma) = 0.0255] Data/restraints/parameters 49547/1385/95223962/1174/1007 Goodness-of-fit on F² 1.091 1.052 Final R indexes [I>=2σ(I)] R₁ = 0.0629, wR₂ = 0.1600 R₁ = 0.0459, wR₂ = 0.1084 Final R indexes[all data] R₁ = 0.0999, wR₂ = 0.1797 R₁ = 0.0720, wR₂ = 0.1241 Largestdiff. peak/hole/e Å⁻³ 1.78/−0.83 1.27/−1.34

TABLE 48 Crystal data and structure refinement for [(NArP₃)CoCl][BPh₄]and (DPB)Co(N₂) Identification code [(NArP₃)CoCl][BPh₄] D(PB)Co(N₂)Empirical formula C63H80BClCoNP3 C₃₀H_(38.67)BCoN_(1.67)P₂ Formulaweight 1049.38 554.31 Temperature/K 100 100 (2) Crystal system triclinictriclinic Space group P-1 P-1 a/Å 10.9491 (7) 10.7517 (7) b/Å 14.9096(10) 18.0184 (12) c/Å 17.8512 (11) 24.6119 (15) α/° 83.935 (3) 69.741(5) β/° 79.063 (3) 81.149 (4) γ/° 89.303 (3) 74.584 (3) Volume/Å³ 2845.1(3) 4302.0 (5) Z 2 6 ρ_(calc) g/cm³ 1.225 1.284 μ/mm⁻¹ 0.472 0.731F(000) 1118 1754 Crystal size/mm³ 0.06 × 0.04 × 0.02 0.27 × 0.17 × 0.08Radiation MoKα (λ = 0.71073) MoKα (λ = 0.71073) 2⊖ range for datacollection/° 2.746 to 59.26 3.48 to 66.26 Index ranges −15 ≦ h ≦ 15, −20≦ k ≦ 20, −16 ≦ h ≦ 16, −26 ≦ k ≦ 26, −24 ≦ l ≦ 24 −37 ≦ l ≦ 37Reflections collected 103727 206469 Independent reflections 15990 [Rint= 0.0972, 29807 [R_(int) = 0.1027, Rsigma = 0.0851] R_(sigma) = 0.1334]Data/restraints/parameters 15990/0/646 29807/949/1005 Goodness-of-fit onF² 1 0.976 Final R indexes [I>=2σ (I)] R1 = 0.0457, wR2 = 0.0837 R₁ =0.0546, wR₂ = 0.1381 Final R indexes [all data] R1 = 0.0986, wR2 =0.0978 R₁ = 0.1214, wR₂ = 0.1552 Largest diff. peak/hole/e Å⁻³0.53/−0.53 0.91/−0.75

ADDITIONAL REFERENCES

-   (1) Brookhart, M.; Grant, B.; Volpe Jr., A. F. Organometallics 1992,    11, 3920-3922.-   (2) Wietz, I. S.; Rabinovitz, M. J. J. Chem. Soc., Perkin Trans.    1993, 1, 117.-   (3) Suess, D. L. M.; Tsay, C.; Peters, J. C. J. Am. Chem. Soc.,    2012, 134, 14158.-   (4) Whited, M. T.; Mankad, N. P.; Lee, Y.; Oblad, P. F.;    Peters, J. C. Inorg. Chem., 2009, 48, 2507.-   (5) MacBeth, C. E.; Harkins, S. B.; Peters, J. C. Can. J. Chem.,    2005, 83, 332.-   (6) Lin, T.-P.; Peters, J. C. J. Am. Chem. Soc., 2013, 135, 15310.-   (7) Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.;    Bourissou, D. Angew. Chem. Int. Ed. 2006, 45, 1611-1614.-   (8) Cotton, F. A.; Faut, O. D.; Goodgame, D. M. L.; Holm, R. H. J.    Am. Chem. Soc., 1961, 83, 1780.-   (9) Evans, D. F. J. Chem. Soc., 1959, 2003.-   (10) Harman, W. H.; Lin, T.-P.; Peters, J. C. Angew. Chem. Int. Ed.    Engl. 2014, 53, 1081-1086.-   (11) Weatherburn, M. W. Anal. Chem. 1967, 39, 971.-   (12) Watt, G. W.; Chrisp, J. D. Anal. Chem. 1952, 24, 2006.

STATEMENTS REGARDING INCORPORATION BY REFERENCE AND VARIATIONS

All references cited throughout this application, for example patentdocuments including issued or granted patents or equivalents; patentapplication publications; and non-patent literature documents or othersource material; are hereby incorporated by reference herein in theirentireties, as though individually incorporated by reference, to theextent each reference is at least partially not inconsistent with thedisclosure in this application (for example, a reference that ispartially inconsistent is incorporated by reference except for thepartially inconsistent portion of the reference).

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions of excluding any equivalents ofthe features shown and described or portions thereof, but it isrecognized that various modifications are possible within the scope ofthe invention claimed. Thus, it should be understood that although thepresent invention has been specifically disclosed by preferredembodiments, exemplary embodiments and optional features, modificationand variation of the concepts herein disclosed may be resorted to bythose skilled in the art, and that such modifications and variations areconsidered to be within the scope of this invention as defined by theappended claims. The specific embodiments provided herein are examplesof useful embodiments of the present invention and it will be apparentto one skilled in the art that the present invention may be carried outusing a large number of variations of the devices, device components,methods steps set forth in the present description. As will be obviousto one of skill in the art, methods and devices useful for the presentmethods can include a large number of optional composition andprocessing elements and steps.

When a group of substituents is disclosed herein, it is understood thatall individual members of that group and all subgroups, including anyisomers, enantiomers, and diastereomers of the group members, aredisclosed separately. When a Markush group or other grouping is usedherein, all individual members of the group and all combinations andsubcombinations possible of the group are intended to be individuallyincluded in the disclosure. When a compound is described herein suchthat a particular isomer, enantiomer or diastereomer of the compound isnot specified, for example, in a formula or in a chemical name, thatdescription is intended to include each isomers and enantiomer of thecompound described individual or in any combination. Additionally,unless otherwise specified, all isotopic variants of compounds disclosedherein are intended to be encompassed by the disclosure. For example, itwill be understood that any one or more hydrogens in a moleculedisclosed can be replaced with deuterium or tritium. Isotopic variantsof a molecule are generally useful as standards in assays for themolecule and in chemical and biological research related to the moleculeor its use. Methods for making such isotopic variants are known in theart. Specific names of compounds are intended to be exemplary, as it isknown that one of ordinary skill in the art can name the same compoundsdifferently.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural reference unless thecontext clearly dictates otherwise. Thus, for example, reference to “acell” includes a plurality of such cells and equivalents thereof knownto those skilled in the art, and so forth. As well, the terms “a” (or“an”), “one or more” and “at least one” can be used interchangeablyherein. It is also to be noted that the terms “comprising”, “including”,and “having” can be used interchangeably. The expression “of any ofclaims XX-YY” (wherein XX and YY refer to claim numbers) is intended toprovide a multiple dependent claim in the alternative form, and In anembodiment is interchangeable with the expression “as in any one ofclaims XX-YY.”

Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of ordinary skillin the art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, the preferred methodsand materials are now described. Nothing herein is to be construed as anadmission that the invention is not entitled to antedate such disclosureby virtue of prior invention.

Every formulation or combination of components described or exemplifiedherein can be used to practice the invention, unless otherwise stated.

Whenever a range is given in the specification, for example, atemperature range, a time range, or a composition or concentrationrange, all intermediate ranges and subranges, as well as all individualvalues included in the ranges given are intended to be included in thedisclosure. As used herein, ranges specifically include the valuesprovided as endpoint values of the range. For example, a range of 1 to100 specifically includes the end point values of 1 and 100. It will beunderstood that any subranges or individual values in a range orsubrange that are included in the description herein can be excludedfrom the claims herein.

As used herein, “comprising” is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps. As usedherein, “consisting of” excludes any element, step, or ingredient notspecified in the claim element. As used herein, “consisting essentiallyof” does not exclude materials or steps that do not materially affectthe basic and novel characteristics of the claim. In each instanceherein any of the terms “comprising”, “consisting essentially of” and“consisting of” may be replaced with either of the other two terms. Theinvention illustratively described herein suitably may be practiced inthe absence of any element or elements, limitation or limitations whichis not specifically disclosed herein.

One of ordinary skill in the art will appreciate that startingmaterials, biological materials, reagents, synthetic methods,purification methods, analytical methods, assay methods, and biologicalmethods other than those specifically exemplified can be employed in thepractice of the invention without resort to undue experimentation. Allart-known functional equivalents, of any such materials and methods areintended to be included in this invention. The terms and expressionswhich have been employed are used as terms of description and not oflimitation, and there is no intention that in the use of such terms andexpressions of excluding any equivalents of the features shown anddescribed or portions thereof, but it is recognized that variousmodifications are possible within the scope of the invention claimed.Thus, it should be understood that although the present invention hasbeen specifically disclosed by preferred embodiments and optionalfeatures, modification and variation of the concepts herein disclosedmay be resorted to by those skilled in the art, and that suchmodifications and variations are considered to be within the scope ofthis invention as defined by the appended claims.

We claim:
 1. A catalytic process for reduction of molecular nitrogen(N₂) to generate a reduction product, said process comprising the stepsof: contacting a transition metal catalyst with a source of protons anda source of electrons in the presence of said molecular nitrogen,thereby generating said reduction product; wherein said transition metalcatalyst comprises a metal complex comprising a transition metal atomselected from the group consisting of Fe and Co, and a phosphine ligand(L) having the formula (FX1A), (FX1B) or (FX1C):

wherein: X¹ is B, C, Si or P; each of L¹, L² and L³ is independently asubstituted or unsubstituted C₁-C₁₀ alkylene, C₃-C₁₀ cycloalkylene,C₅-C₁₀ arylene, or C₅-C₁₀ heteroarylene; each of R¹, R², R³, R⁴, R⁵, R⁶and R⁷ is independently hydrogen or a substituted or unsubstituted C₁-C₈alkyl, C₃-C₈ cycloalkyl, C₅-C₈ aryl, C₅-C₈ heteroaryl, C₁-C₁₈ acyl,C₂-C₈ alkenyl, C₂-C₈ alkynyl, or —P(OR⁸)₂, wherein each R⁸ isindependently hydrogen, C₁-C₈ alkyl, C₃-C₈ cycloalkyl, C₅-C₈ aryl orC₅-C₈ heteroaryl.
 2. The catalytic process of claim 1, wherein saidmetal complex further comprises N₂ and has the formula (FX2A), (FX2B) or(FX2C):

wherein Z is said transition metal atom.
 3. The process of claim 1,wherein said step of contacting said transition metal catalyst with saidsource of protons and said source of electrons in the presence of saidmolecular nitrogen further regenerates said transition metal catalyst.4. The process of claim 1, wherein said reduction product is NH₃ orN₂H₄.
 5. The process of claim 1, wherein said transition metal catalystfurther comprises a N₂ group; wherein said transition metal catalyst hasthe formula: (L)Z(N₂)⁻ (FX3); wherein L is said phosphine ligand and Zis said transition metal atom.
 6. The process of claim 5 furthercomprising the step of protonating said transition metal catalyst so asto generate a hydrogenated metal-N₂ complex; wherein protonating occursvia contacting said transition metal catalyst with an acid.
 7. Theprocess of claim 6, wherein said hydrogenated metal-N₂ complex has theformula (FX4), (FX5A) or (FX5B):(L)Z(N_(x)H_(y)) (FX4), (L)Z(NH₂) (FX5A) or (L)Z(NH₃)⁺(FX5B); wherein xis 1 or 2; y is 1, 2, 3, 4 or 5; L is said phosphine ligand and Z issaid transition metal atom.
 8. The process of claim 6, furthercomprising reductive protonation of said hydrogenated metal-N₂ complex,thereby generating said reduction product and regenerating saidtransition metal catalyst.
 9. The process of claim 1 further comprisingthe step of: providing a transition metal catalyst precursor comprisinga precursor transition metal complex comprising said transition metalatom and said phosphine ligand (L); and contacting said transition metalcatalyst precursor with molecular nitrogen in the presence of an acidand a reductant, thereby generating said transition metal catalystcomprising a N₂ adduct of said transition metal catalyst precursor;wherein said transition metal catalyst precursor has the formula:(L)Z⁺(FX6); wherein L is said phosphine ligand and Z is said transitionmetal atom.
 10. The process of claim 9, wherein said transition metalcatalyst is generated via reduction of said transition metal catalystprecursor wherein said reducing agent is Na, Na/Hg or KC₈.
 11. Theprocess of claim 1, wherein said transition metal catalyst is amononuclear metal complex, wherein said transition metal atom is Fe andcharacterized by an oxidation state of Fe(−1), Fe(0), Fe(I), Fe(II),Fe(III), or Fe(IV).
 12. The process of claim 1, wherein said phosphineligand is a tripodal trisphosphine ligand having a boron, carbon,silicon or phosphorous axial donor atom.
 13. The process of claim 1,wherein said phosphine ligand has an aryl backbone comprising at leastone of L¹, L² and L³ independently comprising C₅-C₁₀ arylene or C₅-C₁₀heteroarylene or wherein said phosphine ligand comprises one or morecyclohexylamine ring systems.
 14. The process of claim 1, wherein saidligand of said transition metal catalyst has the formula (FX7A), (FX7B)or (FX7C):


15. The process of claim 14, wherein said metal complex furthercomprises N₂ and has the formula (FX8A), (FX8B) or (FX8C):

wherein Z is said transition metal atom.
 16. The process of claim 1,wherein said ligand of said transition metal catalyst has the formula(FX9A), (FX9B) or (FX9C):


17. The process of claim 1, wherein said ligand of said transition metalcatalyst has formula (FX10A), (FX10B) or (FX10C):

wherein i Pr is isopropyl, Ph is phenyl and Cy is cyclohexyl.
 18. Theprocess of claim 1, wherein said ligand of said transition metalcatalyst has formula (FX10A), (FX10B), (FX10C) or (FX10D):

wherein iPr is isopropyl, Ph is phenyl and Cy is cyclohexyl.
 19. Theprocess of claim 1, wherein said transition metal catalyst has theformula [(TP^(R)B)Fe(N₂)]⁻, [(CP^(R) ₃)Fe(N₂)]⁻, [(SiP^(R) ₃)Fe(N₂)]⁻,[(TP^(R)B)Co(N₂)]⁻, [(CP^(R) ₃)Co(N₂)]⁻, or [(SiP^(R) ₃)Co(N₂)]⁻,wherein TP^(R)B is a tris(phosphinoaryl)borane ligand, CP^(R) ₃ is atris(phosphinoaryl)alkyl ligand and SiP^(R) ₃ istris(phosphinoaryl)silyl ligand.
 20. The process of claim 1, wherein atleast one of said molecular nitrogen said transition metal catalyst,said source of protons and said source of electrons are provided in asolution comprising one or more solvents, wherein the concentration ofsaid molecular nitrogen in said solution is selected from the range of1×10⁻⁴ M to 1 M wherein the concentration of said transition metalcatalyst in said solution is selected from the range of 0.01 mM to 10mM.
 21. The process of claim 1, wherein said source of protons is one ormore acids selected from the group consisting of: HBAr^(F) ₄ (hydrotetrakis[(3,5-trifluoromethyl)phenyl]borate), HOTf (triflic acid), HX,HBF₄, ArNH₃+X and a combination of these; wherein X is a halogen. 22.The process of claim 21, wherein the concentration of said one or moreacids is selected from the range of 0.01-5 M.
 23. The process of claim1, wherein said source of electrons is one or more reductants selectedfrom the group consisting of Na, K/Hg, KC₈, Na/Hg, NaBH₄ ⁻, Mg, Zn andany combination of these.
 24. The process of claim 23, wherein theconcentration of said one or more reductants is selected from the rangeof 0.1-100 M.
 25. The process of claim 20, wherein said transition metalcatalyst is a homogeneous catalyst, wherein said transition metalcatalyst, said source of protons, said source of electrons and saidmolecular nitrogen are provided in contact with each other in saidsolution.
 26. The process of claim 20, wherein said transition metalcatalyst is a heterogeneous catalyst, where said transition metal, saidsource of protons and said source of electrons and said molecularnitrogen are provided in said solution and provided in contact with saidtransition metal catalyst provided in the solid phase.
 27. A catalystformulation for reduction of molecular nitrogen (N₂) to generate areduction product, said formulation comprising: a transition metalcatalyst comprising a metal complex comprising a transition metal atomselected from the group consisting of Fe and Co, and a phosphine ligand(L); a source of protons comprising one or more acids; and a source ofelectrons comprising one or more reductants; and a phosphine ligand (L)having the formula (FX1A), (FX1B) or (FX1C):

wherein: X¹ is B, C, Si or P; each of L¹, L² and L³ is independently asubstituted or unsubstituted C₁-C₁₀ alkylene, C₃-C₁₀ cycloalkylene,C₅-C₁₀ arylene, or C₅-C₁₀ heteroarylene; each of R¹, R², R³, R⁴, R⁵, R⁶and R⁷ is independently hydrogen or a substituted or unsubstituted C₁-C₈alkyl, C₃-C₈ cycloalkyl, C₅-C₈ aryl, C₅-C₈ heteroaryl, C₁-C₁₈ acyl,C₂-C₈ alkenyl, C₂-C₈ alkynyl, or —P(OR⁸)₂, wherein each R⁸ isindependently hydrogen, C₁-C₈ alkyl, C₃-C₈ cycloalkyl, C₅-C₈ aryl orC₅-C₈ heteroaryl.